I'm a Platinum top laner. Here's my Jayce build!

The Armor pen. is really good early game. It makes your shock blast and acceleration gate combo really strong early game. It is also great for poke.
You would want that dmg because you need dmg on Jayce.
Then you would want both armor and magic resist for against your opponent on the top lane. You would need this so you could survive a bit on top and so you would have a bit of defense.

This is just the average Cdr dmg mastery page. The big thing is the Devastating Strikes since it gives you armor pen. and magic pen. You're Lightning Field does magic damage so it all evens out. Then of course Havoc is good since it gives you that extra dmg.

Jayce's main dmg is mostly your Q and E if you are doing this build.
Don't really need to max ult since you want more points into your skills that are mostly needed.
You could max E second but since it kinda got nerfed, its not as useful as it was last season (Season 2).

You would want to start flask since you use a lot of mana early game. Try to farm a lot until you get enough for a tear. Once you get your tear you wouldn't want to worry about having no mana.
Brutalizer is REALLY good on Jayce. The armor pen. would just nuke your opponent with your shock blast and acceleration gate combo.
Then you would want to get manamune to stack your tear faster and get that extra damage too.
Boots...What is that?
Once you get your last whisper you should just nuke anybody you see. You should so much damage that the other team would say that the other top laner fed you.
Lvl 2 boots...What is that? I would just get Ionian for that CDR.
Black Clever is your only tanky item. It gives you hp and some extra dmg to nuke anybody down if you couldn't earlier.
Once you get Infinity Edge, you should be god like. You would have a total of about ~300 dmg or more. This is where your hyper charge comes to play for your IE. When you crit with one of your hyper charges it should do at least 1000 dmg.
You would want Guardians Angel to be in the front lines and stop acting like a ***** in the back.

You want to farm with your abilities. It helps stack your tear to get marumana. But if you do really bad, you want to freeze the lane. Freezing the lane helps you get back in the game and also stacks your tear.

Once you are in lane try to get lvl 2 first. You get that BIG advantage once you get your shock blast and acceleration gate combo.
Once you get tear, you would want to bully your opponent. Try to get his so low that you could go to hammer stance and finish the person off. If the person starts running away when he/she is low use your acceleration gate to catch up and do your combo.
Try to always play aggressive as Jayce. You would want to win lane and start to roam to help your team and get more objectives.
If you are losing lane, try to stay away from your opponent and just farm with your Q E combo. Also ask for some ganks from your jungler to get back in the game. Once you are back in the game, play aggressive to try to be on top of your opponent and win your lane.

Communicate with your team.
When you are in team fights you should use your poke to your advantage. You should try to aim the Adc, Mid, or support since they are very squishy.
Try to stay in the back to survive in team fights for now and poke. Also, you are really squishy early game. And POKE POKE POKE.
Your hyper blast is really good to kill towers. Use this to your advantage.
Once you get GA stop staying in the back like a ***** and start being in the front. You can use your Hammer stance to try to catch some one out and it also does great dmg.
Don't get caught out.

Jayce is used to poke down and that extra cannon for the team. He is used to aim the squishy's and to kill anybody with armor too with that armor pen.
Altogether Jayce is just really good for nuking down and wining trades.
P.S. Don't build any tanky items, only if need and if someone starts to get pissed at you for doing so bad in your team and saying that you are so squishy in fights and that you are useless.
Then that person will start reminding you about that game for months every time you are in a skype call with that person.
P.S.S Rock that Full Metal Jayce.