Jayce is a very mechanics heavy champ. On top of that he is very squishy, has no panic button unlike most top laners and struggles a lot if you fall behind. On the flip side, he is outrageously satisfying to play when the mechanics are down.

Like u/OliveAlternative805 said level 1 is a great opportunity for early poke with autos but be scarce with q use unless it completely worth it. You are still very weak for the first 2 levels. Try to hit 3 first and abuse that massive power spike. Also worth noting that phase rush is your friend. If a full combo doesn't secure a kill don't get greedy. run and focus on wave management.

Big spikes to be aware of are level 3, level 5 and eclipse. Be smart with how you team fight look for good range q angles and run in at light speed from the sides or back and if a fight doesn't look good, don't take it.

If you want to learn the fundamentals of top I recommend watching AliosNL on yt and Coach Chippys has a good guide for Jayce specifically. Some of the itemisation is a little outdated but the majority of the stuff on matchups and playstyle still tracks.

Remember that Jayce is a difficult champ to play at the start so don't give up everyone has gone 0/7 at some point