How to play a AP Mage at Mid

Just start practicing. I usually find it best when you're learning a new champ to try it out in a custom game first, then a bot game, then go into PvP. That way you can learn the combos and the playstyle before you face someone who actually does stuff (unlike a bot)

Don't pick TF into ahri.

If at all possible watch some videos on how to play the mage you want to try, also when starting out you may want to use ad runes if you aren't comfortable farming with such little damage from basic attacks. But to be honest nobody can really "teach" you how to play mages in mid. Hope this helps a little, and good luck!

If you're fighting an assassin mid, rush Zhonya's Hourglass.

Well, considering Yasuo's current popularity, it isn't the best time to learn someone like TF, and I guarantee you will run into him multiple times while learning TF. He will likely be your biggest problem, but I play Yasuo so I will try to help you out in this matchup:

Playing a squishy mage against , don't attempt to trade autos, push the lane, or otherwise do anything to irritate him or else he will kill you. Only auto him once to bring down his shield, but otherwise just stay away from him and pay attention to your CS and make sure you get every single minion. Use red card to clear a group of low health minions and use blue card every time it's off cooldown, and use yellow card if he tries to get close to you, but DO NOT walk up to him and yellow card him, you just triggered minion aggro and put your only CC on cooldown, now he is going to dash to you and get multiple autoattacks off. Easiest way to deal with Yas is to let him push so your jungler comes to gank, his nonstop dashing in lane would lead you to believe he can easily escape ganks but this couldn't be farther from the truth. If you have a yellow card ready then he should die in that gank or at least forced to flash (make sure you pass the windwall before you throw the card at him). TF isn't required to win lane, all he needs to do is teleport top or bottom once he hits 6 and help his team get a kill. Once your team rotates mid you won't have a problem with Yasuo since he has minimal ranged waveclear, and the earlier you help your other lanes, the earlier your team will come mid and stop Yasuo from farming.

Above all, don't try too hard to win lane with TF (especially if you're new to him), that just isn't how TF works. TF has an easy time CS'ing so under ideal conditions you should be aiming for 90% CS and his roams are among the best in the game, so keep an eye out for other lanes where you can pick up an easy kill/assist.