I just signed up on Gemini, and I have to admit that I have no idea what I'm doing. Is there any place I can study up on how this works?
Investopedia.org is good, in general.
To answer the other part of your question (Market vs. Limit):
A market order is an order that will execute at whatever the current price is (no matter what - so be careful). A limit order is an order that will execute only at the price you specify (or better, so it's much safer).