Why can't I play talon well?

Talon is an assassin. His kit revolves around getting to the back line, and literally 1 shotting the carries, and then leaving the fight. He is extremely good late game, but has a rough early game in my opinion.

Laning: My tips for laning is that he's actually a really good lane bully if you play him right. Just stand in front of your own minions, and make sure you're not taking any aggro. Once you see the enemy is going for any cs. Just use rake, and try to throw an auto attack in there. You will almost always win a trade that way.

You have insane roaming potential with your ult too. You're invisibility, and E can take you to any target in that range without them knowing you're there, and that's what makes Talon better than most mid laners is that he is basically a second jungler, and can burst down carries.

I'm far from being best player, but I can give some advices too. I am Gold IV EUNE atm, mid main, Talon is my second preferable pick after Twisted Fate (but he is far from my main, yes). I may be wrong, let better players will fix my mistakes (and sorry for English. Please, let me know if i have some mistakes with English too) but my experience can help you to start.

• Your Q resets autoattack. Use it wisely.
• Don't engage with you ult unless you clearly know what are you doing (for example if you need some speed boost to chase alone carry when other enemies are dead). Your R can be used as escape. Use E on your victim, burst it out and use R to finish him, to escape retribution from his teammates and to deal some additional damage. I began to do much better when I had started to ult AFTER go-in, not BEFORE.
• Your E has some juke potential. It is not LB's W, but still. Use it on minion to escape skillshoot or jungler. You can use it from brush on enemy chasing you to juke and escape when you don't have ult. This is reason why do I prefer to take it on lvl 2, not Q, that will only help you to farm until you don't have E. You don't need it as hard.
• You are weak until lvl 6 and extra weak until lvl 3. Just try to farm and zone enemy with W. Start with flask, don't be aggressive, your time will come.
• Try to get lvl 3 faster than your enemy. On lvl 3 you can use E>aa>QW>run-away combo. If you got lvl 3 earlier than your opponent, you win trade. If you dodge your enemy spells, you win trade.
• If you play against some paper opponent such as Lux, Xerath, Twisted Fate and other mages, if you harass them before 6, you will kill them once you reach it. If you got new items, you will probably oneshot them on 6 with ignite. Don't afraid to do it. But care cc of course. If you catch root from Lux, you can die fast as well.
• Whatever happens on mid, go roam. If you are wining lane, you can strengthen your advantage and help your teammates if they have problems. If you lose, some kills or even assists will help you a lot. Even if you lose 0-3-0, don't lose yourself. When you can't outplay your opponent or 2 of them (camping is real), you still can kill someone they love. Their adc, for example.
• If you lose hard, go splitpush. You will get a lot of farm, maybe some turrets and distract enemies from your turrets because they will have to choose - to kill you or to lose turrets. With money from farm and turrets you can back into game.
• Sometimes mobility boots are not the best choice. Ionian, mercs or tabi can be better. Use probuilds to analyze what is better depends on your stats and enemy stats.
• Too many defense items - its isn't right. Remember, that your objective - kill carry and leave with your R. Omen isn't best choice. Get hexdrinker if your ap opponents dominates you, mercs/tabi again, qss maybe.
• Don't overextend :) I lost many games cause I overextend and my kda sucks hard (i have 8-9 deaths average, and even if i can kill their carries easy, win tf and game, my stats are awful).
• Just play him. Nothing is better than experience. Or leave him - maybe he isn't your style. I can't play fighters for example.

Good luck :)