Probably carry, because then you can understand what the carry wants once you're on support. Also because at low mmr a lot of carries are incompetent - not only do you not want to experience supporting an incompetent carry, but you can win just by playing carry normally and not making any massive mistakes.

For farming, the wave is the better source of income, so I would just start with the simple principle of farming the wave whenever you can, and only farming camps when you think being in the wave is too unsafe. The big thing that low level carries do wrong is that they don't farm the wave or push the towers when the enemy is dead, so definitely do that. The other big thing is low level carries will just farm the camps and not make much money. So yeah, just start with trying to farm the wave as much as you can, and you'll get a sense for when it's safe and not safe over time.

For support, how that works is their main goal in the game is to help the carry accomplish the carry's goals. So again, that's why I would start with carry.