Welcome to the forums.
How should i start?
Start by reading the FAQ and looking at the forums. There's a introductions thread for this kind of thing.
What should I be doing?
Looking at tutorials, studying professional pixel work, asking questions, and most of all practicing. I recommend you start with Tsugumo's tutorial. Look in the Tools, Resources and Linkage thread. The Recorded Pixel Art thread is also worth it's weight in gold.
Tablet or mouse?
This has been discussed a couple of times. Try searching for the relevant thread.
What should I know?
Follow the above steps and learn as much as your brain can handle. Ignorance is a sin.