I've always had a knack for playing underplayed champions. I played Sivir before the rework, Urgot before M5 brought him back to popularity, so then I moved on to Karma. I played her as a support, and she was ok, but she really got outshined by others. So I tried her mid, thinking I was cool and trolling my team. Turns out she's not bad! 100+ games later, and I've got the build down pretty good.
There are other guides out there that tell you exactly how to play a certain hero. I'm not hero for that - I'm just here to give you some basic pointers, and explain my build. 1) You need to know when to use shield to damage an enemy If your enemy is near an allied creep, shield/mantra the creep and it will do aoe around it, hurting your enemy. But, this will push your lane! You need to be careful. A good opponent will mkeep the lane pushed so you may end up getting zoned or ganked. Just be careful. 2) If you aren't taking much damage, max shield first. If you are getting too low for comfort, max wave first, and heal yourself with it. 3) Get sometime between levels 4 and 8. If you are 100% confident you won't be ganked (good warding), , forget W and max Q and E. Once you get a certain amount of AP and levels on your Q and E you will be able to instantly clear your wave. Walk into the middle of the wave, Mantra/shield yourself, then walk back behind the creeps and wave them all. Clear your wave, then proceed to gank or farm wraiths if there are no ganking opportunities, and your jungler is not nearby / doesn't care.
Magic pen reds should be standard. SEALS are where you can change things up. If you want to be very lane dominant early game, try getting armor seals so you can harass with autoattacks, and take less damage from creeps. If you do not want to get Doran's Rings x2, or if you start with boots of speed + pots, you may want to get mana regen seals. I used to do this but if you get 2 Doran's you won't need to. Ap/Lvl seals are what I usually get. I feel the extra AP isn't much but still gives you some much needed oomph, and if you are the only true ap on your team, you need all the AP you can get. Movespeed quints are BOSS. Get them over 15 extra AP. You need the speed. Combined with your 2% in masteries you should have +6.5% movespeed!
Start Doran's Ring. You won't need the speed if you have movespeed quints and the speed masteries. Your sustain is from your heal, and you need mana to do that. The extra hp and ap is nice too. Get another Doran's Ring and boots when you go back. Try to make sure you can buy 2 wards too. Then, catalyst the protector. Dramatically increases your lane sustain and makes your tankier. THIS is when you can deviate from the build depending on the scenario. I USUALLY get tear of the goddess now, but if you are sure you can grab blue every time (or you went mana regen runes/masteries), finish your Rod of Ages asap. If you don't get Tear, you will have more AP quicker, but you're suffering late game just a bit, and you're dependent on blue. I find the safest route is not to depend on blue. if you do get tear, spam your q all day to build up the stacks. Once you've gotten Rod of Ages, finish boots. Get lucidity boots for CDR. Not all of your spells are always offensive, so it's best to have them up more often than do more burst with each one. Always buy 2 wards when you go back so you don't get ganked mid. Then, build your rabadon's. If you need armor or MR get chainvest or negatron. I recommend rushing rabby unless you are really in need of MR or armor. By now, you should have these items: Doran's Ring x2 Lucidity boots Rod Of Ages Rabadon's If you don't have many stacks on tear, go for either abyssal scepter, void staff, or zhonyas depending on if you need MR, damage, or armor. Once you get over 500ish extra mana from tear, I recommend finishing your Archangel's. ENDGAME BUILD: ROD OF AGES RABADON'S ARCHANGEL'S ABYSSAL VOID STAFF LUCIDITY BOOTS You should have around 900 ap now. Sit back, let your tank initiate, don't get caught, shield your carry or shield/mantra your tanks, heal ALL the allies(for 22% of their missing hp <3) and have fun.