How do you deal with Jayce? What is his weakness? What should i be aware facing him

Please try to avoid counterpicking options, but rather generic tips against him

Refunded my Jayce IP. Feels good.

No tips to be had, watch pro streams, either you're lucky and the Jayce is bad, you survive him, or you lose. Period.
I've seen Yoricks and Rivens survive him.

How do you trade with 6 abilities at lvl 3 with higher base damage than yours ?

This question is a hard one to answer. Because really there is no way to counter Jayce in play.

He kind of does everything. Well I might add.

The only option is to pick a ranged that can safely push against him and can match his poke. Playing a poke game with him is the best avenue IMO. Even then he'll just become a unkillable monster mid/late and not worry so much about towers, minions, or your entire team really.

A tip? Don't let him hit his accelerated Q or get in range of his gap close. Ever. He will always trade for more. Always. And his poke has godly range and hits like a truck.

Avoid those two and you'll at least survive.

Dodging his Q is incredibly important, luckily it's pretty easy to dodge. Gap closers who can stick to him AFTER his knockaway do okay, too, especially with some CC, like Riven, or Xin. Wardens mail is a good item to grab, the regen helps after trades.

I'm not gonna lie though, pretty much everyone has a hard time with him.

I've always had issues against Skarner and Irelia playing as Jayce, why? I'm not real sure.

Good pokers and CC work, too. Just be sure to stay away for the most part and if you're going against him bot, pray you don't get 2v1 on your tower since you can't really push :P

Your gonna want sustain, and maybe a thornmail with magic pen, (if you have any magic dmg to begin with.) to help even out his trades, like it has been said above you need two gap closers at least, (stupid hammer)

If you need a straight up champ, i think nunu should be able to survive (not sure early game).

You have to remember that Jayce has the two modes and keep in mind that when he shifts, he has locked out half his skillset for 6 seconds.

The practical upside of this is that as soon as he goes from hammer to cannon mode, HE CANNOT KNOCK YOU BACK! This is when you use your gapcloser and beat the **** out of him. He has his Qball which he can't really augment when you are in melee, and he has the attack resetter(hypercharge). Its a long 6 sec until he can escape you and if you have any kind of slow then you will negate his Gate bonus. Besides by the time he gets way, youve already gapclosed and beat him up decently.
"but he stays in cannon." Not really. He likes to shift to refresh his armor pen aura and use his Aura/Gapcloser to push creeps more occasionally. Wait for him to do this if you are a gapcloser. Punish him.

Try NOT to use your gapcloser when he is in cannon form. You ideally want to just walk up to him and attack, or approach thru the bushes. He will simply go to hammer, Slow and Knockback on you. Depending on who you are, you probably not be able to win the damage exchange by Gapcloser/AA/attk resetter/whatever vs his pointblank Qball,possible Hyperstrike, and then his Gapclose/Knockback.

Most Jayce will open their gate before Qball and thus telegraph what they are doing. As a Jayce I rarely shoot AT the enemy, but at the non-moving creep right next to them. Due to the big AoE that is more reliable than a direct shot...unless I am attacking them at the max range of 1700ish where it blows up anyways.

He is relatively easy to kite. 500 autoattack range. Any Ashe, Cait, and many spellcasters like Ryze,etc can poke at him pretty easily. Their stuff is a lot more accurate. Gated Qball is fantastic vs creeps and at really long range, but is ironically hard to hit with at short range and has a 12 sec cooldown. Other than that, ALL he has is the 500 range AA.
Dodge the Qballs which are predictable usually, and poke away.

He might get mad, goto Hammer, Gapclose on you. But all he has is the **** aura, autoattack, and his Knockback (which isn't exactly a great move vs a ranged kiter). And hes locked out of his Gate/Q/Hyper/range! Any kind of blink like Ez, Graves dash, Vayne knockback, whatever will totoally hoze him. He will be unable to do ANYTHING for 6 entire seconds.

They kind of gave that one waaaay too much in his kit like they did with Irelia. Powercreep to the max.

You can't trade with him.
He has outstanding poke.
His mana costs are low, and he can regenerate mana if he's low.
He has %health damage to deal with tanky targets.
Is incredibly difficult to build against, and he doesn't have as many issues with AS slows as other ranged physical champions would have thanks to his w
His escape ability is outstanding. 2 different possible movement speed boosts, an aoe slow, and a knockback
He can go ranged if he's over worried about retaliation, and can simply poke the enemy champion down until he's comfortable again

The only thing they didn't give him is health sustain (and an ultimate trololololo).

That being said, try being VERY map aware. If he's going to get ganked you need to be on the ball and coordinate that with your ganker as much as possible. He WILL get away if you aren't ready for the gank. His acceleration gate will also help you predict when it is that he's going to poke you. Try ganking him after hes used that so it takes away one of his escapes.

That being said, don't fall for the trick where he casts q before acceleration gate. It's a little tricky to pull off, but you might suddenly take a shock black to the face that looked sluggish and easy to dodge.

A properly-built Jayce is essentially a hyper-carry of monstrous proportions.
It's nearly impossibly for most champions to kill him 1v1.

The best thing you can do against him is either 2v1 him, or send a burst assassin to take him out in 1 shot.

I've seen late-game Jax do well against him, as well as Sivir (surprisingly).

However, you're not looking for champion counters, you're looking at what happens if you blind pick and are against him. Honestly, the only thing I can think of is to farm from a safe distance. If you think for one second you're in range of anything, back off - you'll lose have your health bar before you know what happened. This is bot lane strategy. No clue about top lane.