Heyo everyone! I have been trying a lot lately to learn Jayce, but I really do suck with him in every aspect. The main things I want to know are as follows: -How do you build him? -What abilities should I max? -Should I be really aggressive or passive in lane? Any positive feedback is appreciated. Thanks!
Go aggressive in lane, you have ranged advantage and have your knockback and acceleration gate to get you to safety should things get too heated.
He is the quintessential AD caster, so build a mix of AD, armor pen, CDR, and a mana item. 
are all items that he likes.
He falls off rather hard late game, so your goal should be to get your team ahead and close out the game ASAP.
All the good Jayce's I see start combat in ranged mode to get of his e-q combo. Once and only if that lands, you follow up with w-r-w-q, saving E for if you need an escape or to zone.
Essentially that combo breaks down like this, you lay your gate and shoot a charge through it to get the first burst, then you activate his steroid, and then switch to melee to activate his aoe. You do this while running through the gate and suddenly you have crazy gap closing and dps potential due to his passive and r passive (which deals bonus magic damage for the first hit). You then use your q to continue your gap closing and lay a CC and to follow up the second part of your burst with three quick melee attacks, while the aoe rounds off the damage. At that point, they should be low enough that you can finish them off, force a retreat or then you use E to force a disengage to set up for a second volly. You can also pursue them in ranged mode to get an extra aa or two if they retreat. This is an ideal scenario though, but that's the jist of how his combo works.
full damage get to late game 1 shot enemy ADC
In the lane phase don't waste mana through harassing with abilities , just auto attack and wait for some levels so your abilities become strong enough, its very hard for new jayce players to manage their mana pool, that's also one of the main reason jayce is such a high skill cap champ , later in game you play and position like an adc , you should only go in hammer form when the enemy is running away or when you are are you can get out alive, if you are not experienced with the role of marksman you will find it hard to perform well with jayce lategame unless you are fed . as for the build , I think the recommended build is just fine , don't bother your self with builds and items.
