I'm currently learning Mid. My least comfortable role because of so many positions I can be ganked from. So far I can only mid with 
. Any other champs that I should get to mid with? Any general (positioning, when to go for the kill, and when should I roam) mid lane tips?
I ward both sides of the river, farm safely, poke when possible, focus on farming, and roam to other lanes when their mid isn't trying to push my tower.
There are lots of choices really. Any assasin or Mage will do well mid. Just try them out and find another you like.
If you have good wave clear, if and when they leave push the wave to the tower and roam. If they roam and you can clear the wave clear the wave and follow if you can. Make sure to ping if they go missing they might be roaming if another lane is pushed.
Oh those small single bushes on either side of the lane, drop a pink in one of those. It helps a lot sometimes.
As you get to higher levels, you will see a lot more assassins being played. First of all, they will all be incredibly mobile and annoying due to their massive burst damage and mobility. Just stay back , try to let them push wave, and attempt to poke them. However, if you start losing lane or if you can convince your top laner, it might be better to have top go mid and mid go top, as mages can generally poke and annoy the tankier top lane meta which usually builds armor. In the same aspect, tanky top laners will be able to deal with bursty ad assassins better, such as zed, kassadin, talon, sometimes fizz, and ect.
As on the subject of roaming, it greatly depends. If you are crushing you enemy mid laner, then it might be best to roam after you kill them if you still have good health and have the wave pushed to their turret, roam. Also, if you can talk your jungler into it, pulling a two man gank bot and turret diving the enemy adc and support can usually give you two kills and a tower.
Another tip I would give, is to never just have the side bushes warded. You will have vision, but that vision is extremely limited and if the enemy jungler is someone like voli, warw, vi, udyr, or hecarim, who all have amazing gap closing, they will beat you back to tower. Like blue said, get a pink war in one. Also, at level 9, upgrade your trinket so that you have 3 green wards to set down. Even though you will have to go back more often to replace them, these won't be seen unless the enemy has a sweeping lense and still offer good vision.
One last tip, is that when the enemy is hiding under tower and you are just sitting in lane, go for the enemy birds or for your own birds if your jungler isn't clearing right then and there. It gets you gold and exp to help you get ahead.
keep going with malzahar, hes pretty strong right now and he beats assassins other then nidalee and maybe talon(idk about talon never played or seen that matchup)
Warding both sides is expensive and might put you back a bit in terms of gold. You would be better off warding one side and "hugging" it so that if a gank comes from the other side you have time to react. Also, don't push lane until you are strong enough to roam.
If you like Malzahar, you might also enjoy long-range waveclear mages, like Xerath and Ziggs. They don't have the fun instagib combo Malzahar has, but they do have extremely good poke and can destroy enemy minion waves with a two-spell combo. If you prefer the wreck-your-opponent's-face aspect of Malzahar, you could try LeBlanc. Swain has a similar pattern of dropping everything (well, Q and E on the enemy you catch with your W) on one person to kill them, but he's more of a walking juggernaut of bird-based spellvamp than an assassin or waveclear mage. Karthus is similar to Swain once he's farmed up, but has more niche mechanics (Q skillshots and ultimate timing, as opposed to Swain's landing W and wading into the fray as a frontliner).
Thanks everyone. Keep the tips coming.
If you get pushed to your tower and can't get out to ward you can throw your ward over the walls next to river.
After 6 you should always try to find a kill every time you have R ready.
