Kart & character selection
This is very basic but it's one of the most important aspects of online play. To give yourself at least a sporting chance you should avoid all heavy characters such as Bowser, R.O.B, Donkey Kong, etc and avoid all the big-wheeled karts such as the Cucumber, the B Dasher, the Royale, etc. Online games are hectic with a lot more weapons flying at you than in a single-player game so you need to be able to recover quickly; choosing a smallish character and a quick, agile kart will help you. If you plan to use the snaking technique then you should use either the Dry Bomber, the Poltergust 4000 or Egg 1 and choose a lightweight or middleweight driver like Toad, Dry Bones or Yoshi.
Course selection
You're never guaranteed to get your own way every time when it comes to choosing a race track - but there are certains things you can do to make it more likely. The most common way is the wait until all other players have chosen their track and then make your choice based on that. Say, for example, your 3 opponents have chosen Baby Park, Mario Circuit and Wario Stadium, simply choose your favourite out of these 3 tracks and it is guaranteed to win the vote. The only problem is if other players try to do this technique you'll run out of time and end up with a random track.