I play Karma on and off in bot, mid, and sometimes top, but I mainly play her in bot lane so I cn help you there:
Get spellthief, 3 health biscuits, and one mana potion (biscuits and the extra potion comes with putting points into the utility mastery tree, you might not have access to this just yet). 85% of the time I would not start a with a ward. The reason behind this is it will force you back to base after one engage or a bad trade. Having a potion advantage is huge because it allows you to stay in lane and get health advantage over the enemy. Sometimes I get a ward if the other team has a good level 2-3 gank jungler (like lee sin or even kha) but this is much less likely to happen now with the preseason jungle being a lot harder to clear. As Karma you should be aggressive early on because Karma has one of the best early games in league, so you need to take advantage of it. Poke as much as you can, but don't go out of position (ie. facetank enemy minions) just to land a Q. Also, remember that her Auto Attacks lower the cooldown of Mantra (R) if you use it on champions. If you see the ther ADC going for a last hit on a minion, that is a good chance to land an AA or two, provided it isn't risky. You should especially do this against melee supports like Taric, Braum, Leona, etc. Sona is a really bad/hard matchup for Karma because she can poke just as well as Karma but has sustain. Here I would just try to push to level 2 faster and then go hard on her. with R, Q, and W, Karma's level 2 is pretty good and Level 1-2 Sona doesn't have any CC at all so you should be safe if you're at decent health. Soraka is kinda difficult too, but the key here is not to target her lane partner so much but instead try to poke down soraka herself because her heal actually costs her health. Use the movespeed boost on E to dodge her starcall because it is the only way she can continue healing herself. Leona isn't too bad for Karma - but you want to do the opposite in this match up and not target Leona but instead target the ADC instead. Leona can't jump on you or your ADC if her own ADC is low health because then you can just pop the ADC and Leona isn't that much of a threat on her own. If she engages (look for when she puts up her W eclipse ability, it usually means she will jump on you) your best bet is to shield and use W to disengage.
For Skill order, I take Q first, W second if I get level 2 first, if not, I level up E. If you think you're getting ganked soon, pick up whatever you didn't take at level 2 at for level 3, otherwise put another point into Q and wait for whatever you don't have at level 4. I max Q first, then E, then W (when supporting)
For build I generally upgrade to Frostfang first, buy a ward and a few potions. Next back I get sightstone and tier 1 boots, but above all, it's important that you swap out your yellow trinket for a red one once you get sightstone. I probably get chalice after that and then work to Frost Queen's claim or tier 2 boots. The next is situational. Don't upgrade to ruby sightstone immediately. Buy oracle's lens (upgraded red trinket) when you have the spare gold. Try to always have a pink on the map, especially if you are winning. ALWAYS have one on hand when up against

and to a lesser extent,
.
If you're ahead:
(I generally dislike picking up athene's, there is almost always some useful CC to get rid of on the enemy team.)
If the other team has a lot of magic damage / Crowd Control:

Have a lot of AD on your team? Do you have
on your team? (and other champions that like Attack Speed?):

Ardent Censor is good on Karma, but only if you team can take advantage of it. If you have like Vlad/Rumble top, Lux Mid, Sion Jungle, and Ezreal ADC, Ardent Censer probably isn't a good pickup.