ITEMS

Kraken Slayer can be built before full boots if you can afford it on second/third back, but most of the time it's better to go for boots instead.

The extended range from Rapid Firecannon is necessary in order to kite enemies, especially if they outrange you under normal circumstances. At the point of the game where you can afford it, your passive will provide enough slow in combination with your ghost summoner spell so you can both outrun and outrange enemies permanently (Doesn't count for every adc though).

Infinity Edge is the next plausible item, as it provides you 35% more damage on your Crits - Necessary to keep up with the damage of other top laners and to stay useful beyond supporting your team with W-spells.

BotRK gives you max health damage and makes you even more able to shred tanks. In addition, you gain a bit of sustain through its lifesteal, Attack Speed to proc your pta rune earlier and more burst through its magic damage passive.

Lord Dominik's is important if the enemy team has at least 1 tank and bruiser, as you won't deal as much damage to them even if your pta proc went through.

Lord Dominik's can be replaced with Bloodthirster, Guardian Angel, The Collector or Spear of Shojin (haven't tried that yet) if needed.

RUNES

PTA is hands down the best rune on Bard top lane, as you can do unexpectedly much damage to the enemy when it procs, not only in laning phase but also in mid- to lategame.

Triumph is important to keep Bard alive in teamfights, as he is insanely squishy and can easily die if cc'ed.

Alacrity gives you attack speed, that is necessary to proc pta faster in laning phase - it can, however, be replaced by the tenacity rune if you play against champs with a lot of cc.

Coup de Grace is important to get kills in laning phase, as Bard will already do more damage because of the pta-proc. In addition to pta's damage, you will deal more damage to the low health enemy, making it possible to dive them a bit with triumph.

Celerity is important for roams and to outrun and kite enemies more easily.

Paired with celerity, the Waterwalking rune makes it possible to contest objectives, especially rift herald and helps you in teamfights in mid- or lategame.

The last mini-runes can be adjusted according to the matchup - the attack speed is nice to have for good trades in laning phase though.

Collect your passive stacks before the minion wave reaches the middle of the lane, but leave passive stacks that are close to your lane, as they will be an important mana source (you will run out of mana quite often if you or your enemy are playing aggressively in laning phase).

Ult the enemy tower if you (and your jungle and/or mid) want to dive the enemy, as it will stop the tower from dealing damage to one of you. It can also be used to reset tower aggro, if one of you is dying to the tower.

If your wave is pushed, use the opportunity to

collect passive stacks

look for a mid lane gank

look for a gank on top with/without your jungler

After you went to base, you can look for ganks even on botlane if your wavestate lets you do it without losing too much tower platings and gold/xp.

At the game start it is important to know that Bard's passive will spawn on botside if Bard doesn't move out of the fountain the moment he's able to, so don't go to the bathroom if you're in loading screen!
Note: This also means, that it will not be worth for you to join your team when they invade the bot enemy jungle lvl 1, as your first 3 passives will spawn on botside if you do.

Don't use your Ultimate if you could hit one of you teammates, especially if they are using their own Ultimate (e.g. Miss Fortune, Xerath, Karthus).

Don't sit there with more than 2000 Gold without going back to base, as you will fall behind really quickly on Bard if you don't spend your gold on items.

In Early Game, you can easily 1v1 champs if they have 50% hp or less, as you will deal more damage with pta and coupe de grace. To keep up with the enemy if they use flash, you can just ghost and flash as well.
So after you hit lvl 3, you are free to do whatever you want. If you have a supportive champ on midlane (e.g. Seraphine or Karma), it will probably be a better idea to stay top lane and look for gank opportunities to feed your jungle. To have an easier laning phase, it will still be good if you are able to leave lane sometimes, as you will need 15 stacks on your passive to get a good wave clear early on (at 15 stacks, your meeps shoot a wave behind your auto attacks that deal a bit of magic damage and slow hit enemies or minions/monsters).
However, if your midlaner picked a very snowball-heavy champ like Katarina, Yasuo or Kassadin, your help can be very useful to bring your midlaner ahead early on, and possibly making it easier for them to play against a bad matchup for example.

Although Bard is a roaming champion, your priority in mid game is, to push waves on sidelane in order to prepare for objectives. As you are on a sidelane, you will be able to collect more and more passive stacks on bot and top side jungle. At this point, you can also look for a 1v1 in river against an adc, support or jungler, as your runes will provide you an extra ad boost, making it able for you to win a 1v1 against either of those 3 roles.
In Teamfights, your role is to ult the enemy if they try to run away from or engage on your team and afterwards stay in backline to proc pta on a priority target (most of the time it will be the enemy adc, mid or jungle) and to deal damage to them.

In Late Game, your passive stacks are high enough to 1v1 every champion if you play correctly and kite them well enough. 1v1's against fed adcs are to be afraid of though, as they will outdamage and outrange you in 99% of fights.
Your goal is still to push waves and take towers, as your AD and AS are high enough to get towers with low health. The pushed waves make it possible to put pressure on the enemy to finish the game or to get objectives.
If you fell behind, try to collect more passive stacks and look for kills in river to get your team back on track, splitpushing/pushing side waves is still important even if you're behind.

Bard is a great top laner to play for your team and have impact. It is important to keep in mind that he is very squishy and will be outperformed by many splitpushing champions, as he can't match them if you want to have more than 2 passive stacks per hour (It is very important to have those stacks to keep up with enemy damage). You have a lot of impact on the game and can feed snowball-heavy champions in your jungle and midlane and the runes make up for the missing teleport summoner spell.
All in all, this lane is hard to play and hard to master though, as you need to be able to spot enemies early so you don't get catched and you need to kite enemies in order not to die against them. If you get ahead though, your impact on the game can be higher than a lot of meta toplaners, that won't be able to have an impact due to the unleashed TP update and high cooldowns on it.

PRO

You have a lot of impact if you play Bard top lane correctly

Especially paired with a premade in mid or jungle, this pick can make you climb ranks pretty fast

You deal a lot of damage and outrange every top laner in laning phase

Snowball-heavy champions benefit a lot from your help on mid or jungle, as they are able to come back against bad matchups for example.

CONTRA

You are extremely squishy and prone to getting killed by assassins and fed adcs

Once you're behind, it's hard to come back, as you're both squishy and deal no damage - you might want to return to play Bard more as a supportive champion if you continue to feed the enemy if you continue playing as before

Splitpushing champions match your impact in the game and make it hard for you to win the game, as you can't collect passive stacks or have impact if you have to defend the enemy top laner

I want to learn Bard, is top Bard a good way to learn him?

Bard top lane is not the best way to learn this champ, as he is easily punishable because he is squishy and falls behind very quickly, making it hard for beginner Bard players to come back and have any impact on the game, as they don't know where to stand in teamfights and how to react to different situations (e.g. in 1v1's and which lane to go when backing if top lane is pushed out).