When I join a ranked team I am thinking about maining mid. Was just wondering what I should be focusing on?

Last hitting is the best start.

And that is one of the things I am bad at.

Bump

It is important to make sure not to scare away your prey. Guess how much damage you can do in a maximum burst, and how many harassment attempts you need to get your opponent that low. One thing I often did as Veigar is to send a few Q's at my enemy until they were about half life, where a stun-meteor-Q would land me a kill. Since all of your damage is cooldown based, you really only get one shot, unless you have extra damage spells on hand (but no real AP is going to stick around to let you burst them).

Understand that your opponent is counting on this just as well. This is why Annie is a well-used mid. She has the element of surprise with her stun, and with proper damage judgement, she can melt you after a few harassment trades without really giving you a chance.

Understanding how to take care of certain champs in different matchups is very important. Against Akali or Fizz? Be aggressive as soon as you enter the lane. Lux or Fizz? Get early boots and stay frosty.
Sometimes, you might be dealt a losing hand, and you may just have to swap with your bruiser. Khazix, Vi or Talon are well known to go mid and destroy stupid and stubborn AP carries who refuse to move for the sake of the 'Meta', but they are all usually well countered by bruisers.

Last hitting, as Nine said, is very important. Want to practice last hitting? Play as Leona. One of the slowest attack animations with low base attack makes her one of the best to practice with.

Learn how useful breaking the Line of Sight is. Whenever you have your lane pushed, act like you're going to gank by just moving to a side brush. This makes people believe you are missing to gank. Repeating this process will mean that their MIAs (or SS's) are spamming the chat and have little value, or they'll just stop calling it. Champions that scale well with Athene's (Ziggs, Lux) can usually melt the minion waves with no problem and hide, causing tons of chaos for the enemy team.


I'd honestly have to say that the most important part about playing an AP mid is knowing your enemy. Know what Veigars do when they try to initiate with a stun. Know the telltale signs that Akali is itching for a dash-burst. When Annie is just looking for an excuse to summon Tibbers. You'll be able to tell with experience. Veigar's stun is honestly the best example of this, and it's really quite obvious. It's unfortunate he's rarely seen nowadays.

How could one not recommend practicing last hit with Karthus XD

I would say if you get countered, roaming will likely work out better for you than lane swapping, because the enemy team will probably just lane swap themselves. Push your lane as safely as you can and keep good eyes out for helping sidelanes get kills. This way you pretty much avoid your dreaded mid matchup and still get fed.

What is more useful is timing your faster kills with a last hit on minions. In other words, auto attacking AND getting last hits. This entails knowing what your auto damage output is against the 3 different minions, paying attention to how many minions there are attacking the one you wish to last hit, and timing your attacks so you don't just stand there and wait to last hit, but you help wittle them down faster and get a last hit as well.

The benefit to doing this is that a) most champs won't try anything against you because you are surrounded by minis who can absorb the directed blows against you b) if they do, you can back out as they approach c) because you push their turret first by lvl 2, their minions and champ will counter push. You are able to force the opponent to push out past mid, letting you, by lvl 3, start hurting them to keep them away from exp. This is also a low enough level where their jungler, if they have one, will not be there to gank you. Therefore, you can harass all you want and NOW concentrate only on last hits so that the minions stay closer to your turret for longer, allowing you to keep the other mid away from the leveling.

The key is to kill the enemy champ. But if you can't do this, you must keep them away from xp, either by hurting them enough and forcing them to back off, or blocking their desire to enter close combat with the minions.

Thanks for all the help, I have been enlightened.

Unless you only do support - that's essential for all the lanes.