Sorcery should be your main tree line preferably with comet as it gives you a higher damage output. You can trade out for aery if you want a bit more survivability. Scorch is good for the extra damage but you can change it for storm if you want some scaling factor.

Inspiration for you second tree with biscuit and insight. Pretty standard for most builds bot lane. Gives you extra haste plus free tiny pots.

Just the basic flash ignite for escapes or engages.

Early items:

You can replace Spellthief's with Relic Shield if you want to help bot with wave control but your passive helps you gain gold much faster on Spellthief.

Your Main Kit:

Imperial helps give you extra damage on your slows and burst for your team from the mark.

Chemtech Putrifier is great for stopping heals throughout the entire game so you want to build it in parallel to Mandate or even buy into Oblivion Orb early.

Rylai's gives you stun on every e regardless of having the passive double strike up.
Once you build into all of these items you can pretty much handle every time fight if you stay close to your team. It's even easy to get tons of solo kills if you are good at kiting and using your passive on the right abilities.

For the boots you almost always want to go Lucid as it gives your summoner's a faster cooldown for more ignite plays. Sorcerer's are viable if you don't have a carry and are dealing lots of damage for your team.

For the situational items:

Flowing water staff if you want a speed boost for fights.
Ardent for extra damage.
Zhonya's for survivability and more often than not you will want this for the extra armor and stasis.

High note: good for zoning and harassing enemy bot to keep them from feeding or from having favorable trades. It has a small aoe which lots of people tend not to dodge, especially in lower ranks. You want this to be your first maxed ability as it is your main source of damage in conjunction with your passive.

Surround Sound: Has long cooldowns so will be your last ability to max. If your laner is taking heavy damage you can opt to take this level 2 instead but most likely want to take your stun potential in the E. Good for engaging and disengaging and will give you tons of maneuverability the more teammates are near you.

Beat Drop: This is the bread and butter for early picks and in most teamfights. Early game you have to rely on your passive to get the stun off unless your laner has slows in their kit. Once you build into Rylai's it is an automatic stun every 5 seconds or less depending on your chosen runes. Use this after your ult to maximize damage during teamfights or to keep the enemy away to let you escape. If you shoot your Q and then E right after it is a guaranteed stun with Ry.

Encore: Basically an Ahri charm but with a Sona style. Best used in conjunction with E to give your team the best chance at winning fights. Not much to say here other than planning the correct time to use it. Great in jungle corridors as there isn't really anywhere to run to avoid it. Great for ambushes or for prepping teamfights to your advantage.

Seraphine is extremely fun to play and great in teamfights or laning. If you practice with her you can get stuns off all the time to give you teammates the best chance at snowballing and praising you. I have had multiple games with tons of assists and kills with this build. Try it out for yourself and see what you think. See you on the rift!