For this Jayce build, I have a good solution for a surprisingly common problem; not having a proper tank. If you don't have a tanky support or jungler, I would suggest this build. For me, I like to play rather aggressive, so I build this anyways. This build has very high resists and STILL you will deal high amounts of damage. This build also takes care of Jayce's infamous mana problem.

You can switch out the Guardian Angel with a Mecurial Scemitar, Blade of the Ruined King, Infinity Edge, Spirit of the Lizard Elder, or even a Frozen Mallet, whatever calls to you. If you have the gold, you CAN switch out the Maw for a Scemitar if you don't feel the need for a spell shield. It has higher defensive and offensive stats than the Maw, so it's worth it.

Use your Q,then while it's on it's way, E in ranged form so that you enemy doesn't see it coming nearly as easily. You will then waste less mana! Also, you can use acceleration gate, switch to hammer form, run through your enemy, and then knock him into your team in engagements to secure their death. You should also use your gate to give the speed buff to your team for chasing, then switch to hammer form, To the Skies!, slowing their team down, potentially securing more kills and helping to the victory of a teamfight. Also, especially don't forget your passive on your first basic attack in cannon stance. End game, it reduces an enemies armor and magic resist by a drastic 25%, really hurting their resistances. Of course, these are just a couple of many tips, but ones that will make the pro plays.

Pros-
Very strong champion
Has some CC
Deals damage and can tank
Can assist team in escape/chase

Cons-
Probably will get focused
Maybe not as much damage as you'd like?
(If you don't feel you're dealing enough damage, then damn, you want to be adc)

I take heal for help if I need it in an engagement, if I get ganked, or if our jungler ganks and we need the help. I then take flash for the escape, since Jayce already has plenty of speed buffs.

In your top lane, since you have a transform, you can usually easily and safely farm from a distance. I prefer this way of farming because you auto-attacks have a very quick response time. To keep up your mana though, if you can without taking much damage, just farm in melee (remember, you have mana per hit regen)

I choose these as my runes so that you can actually get in the battle without having to pay for it all that much. You can switch out the quints if you'd like, I just know that my build lacks health, so I have those to make up for some of it.

Once again; defense. To deal damage, since half of it is in melee, you have to get in the middle of the fight. I took the mana regeneration because, well, of his mana issues.

It's just a tad situational, but I always take my Q first for the poke and high level one damage. I max it for the mental poke and melee damage. I then max my E so when champions get too far into my comfort zone, I can hit them away for a large amount of damage. It also allows for more spamable Q-E combos, and higher escape ability. I take W last because, well, it doesn't deal much damage.