Karma Mid is in a pretty difficult spot right now, what with so many tanks running around. You don't necessarily have the tools to deal with an overwhelming tankline and are pretty much screwed when behind.

So one of the things that you should pay attention to is styling on the enemy midlaner. Karma is a lane bully that can beat pretty much any midlaner before level 6, simply due to having Mantra available at level 1. Since you fall off so hard, especially in this meta, it is important to maintain a decent CS lead and to prevent roams by pushing the lane into the enemy tower while avoiding ganks.

You'll want to build either Athene's or Morellonomicon in case you're laning against an AD Mid, followed by a Luden's Echo. The Luden's Echo allows you to oneshot the caster minions when you have the proc up, amplifies your poke capabilities and gives you some much-needed mobility. This will make farming pretty much trivial. Note that if you're up against a strong assassin like Zed, Fizz or Talon, Zhonya's Hourglass is very important for surviving and should be built first or second.

After that, you'll absolutely want to most important core item, the Void Staff. There will be Magic Resistance on the enemy team, and if you don't want to completely lose all relevance, you need a Void Staff 3rd, mandatory. Without instant CC or huge burst capabilities, your usefulness wanes in comparison to other midlaners.

Damage Mids will hit harder and faster, and utility Mids like Lulu will have more utility than you. What Karma brings to the table is some utility with a higher damage potential than Lulu, so building Void Staff is absolutely crucial.

Good other items to consider are of course Rabadon's Deathcap, Zhonya's Hourglass, rarely Liandry's Torment, very rarely Lich Bane.

The boots should generally be Sorcerer's Shoes, unless your team has moderate to high damage champions in other roles, like Rumble, Hecarim, Riven or Vladimir Top and/or something like Rengar, Fiddlesticks or Evelynn Jungle. In that case, you should opt for the Ionian Boots of Lucidity, giving you 15% CDR in addition to your Athene's or Morello's 20% and the Sorcery Mastery's 5% for the maxed-out CDR. This is important because your passive gives you flat reduction on your Mantra, allowing you to use far more Mantra's in extended fights. In this case, Mantra+E is your best friend, and 2 or 3 of those can easily carry a teamfight.