Hello looking to buy a new ap mid. Mid is my weakest role and I would like to buy a good champ that I can play in ranked, not many counters etc. My current mids (that I can play well) are

Morgana
Kennen

also Katarina and Mord (but they are kinda situational)

Also any general tips for playing mid lane such as when to gank bot and top etc-as I said mid is my weakest role so I would like some help to play mid better

The absolute safest mid pick you can get is Galio. He doesn't really have a counter and he gains ap by building mr. His shield can make a burst tickle he can heal off minion aggro and his ult is a complete game changer.

Although most people don't care for his skills as they feel "sluggish or boring". Give him a shot on freeweek he's up there pretty often.

Also try cho gath, his hp regen for killing minions allow for insane lane sustain.
For a build, I found a decent one recently which allows for HP defense and offensive capabilities.
Rod of Ages
Boots of Swiftness
Frozen Heart
Athene's Unholy grail
Rylai's Crystal Scepter
Spirit Visage.

This will change for the season 3 patch as builds will be changed alot. But its decent. end game have about 4.1k HP 1.7k mana 320+ AP 190-200 armor 120-140 magic resist depending on perlevel runes or standard straight runes.

Just went 18-8-18 with the most gold in the game.

Another pretty safe mid you can pick is Vlad.

Easy farming and sustain with Q, escapes with W, etc. You can harass and trade, but you can also play passive and just farm under turret with E if you're pressured as well, meaning he has a nice crutch in his kit to rely on to get used to.

Besides, you can always play top with him if you can't lane mid. :P

Elise is a very safe mid with good damage output throughout the game due to her ease at escaping. Also because she's new, not everyone knows how to deal with her

As mentioned by whoopsi, galio works well mid as he is an anti ap tank. If you build his shield as the second choice after his Q, you will leave all engagements with more health than you started, not to mention making the adc/focused team mate an unstoppable tank during any early team fights

Ahriis hasn't seen much play lately, but she is really fun and fast-paced if that is more your style (think kennen).

If you like Morg, you could try Lux. Similar kits but Lux has a different playstyle.

Lux isn't really like morg. Morg has a game changing ulti, lux has a nuke of an ulti with a really short CD. I play lux uber aggressively. Usually I can out damage and outzone the other mid champs, other then fizz. But even then the power poke then burst placement is enough to send anyone packing. Get someone down to 500 hp at level 4, e,q,auto attack, pop e. Auto attack = instant nuke. Its pretty insane burst with such long CD. I've even gone 1v1 against a brand that had to get Rylai's + Giant's belt + Rod of Ages to counter the sheer power that lux and put out.

If you're willing to put a lot of time into it. I feel like karthus and orianna sort of have infinite skillcaps, and when played well are insanely lethal. I like karthus more because if you're focused in teamfights you can still put out damage after you die

Karthus is a really safe pick because of his long farming range and his great late game. I will first pick him and it's really hard to counter him. I usually spend the first 20 minutes farming and maybe trying to gank bot, while assisting ganks with my ult. Then once he has RoA and Rabadon's he's brutal in teamfights.

If you want to be the only BAMF with Mejai's Souler in your Season 3 Recommended Items, pick Lux. The fun is strong with that one.