Everyone's first cup is a shambles
Give it two cycles before you decide anything. The first is a learning curve for everyone, including the people who say it wasn't. By the second, you forget it's there.
Which size
Size 1 Regular is where most people start. Size 2 Large suits heavier days and anyone who's given birth. Between the two, start at Regular.
How long can I leave it in
Up to eight hours before it needs emptying. A work day, a flight, or a lunch that got away from you. Not longer.
Will I be able to get it out
Yes. Bear down, pinch the base to break the seal, take your time. Panicking is the only thing that makes it harder.
How do I clean it
Rinse between empties. Proper wash at the end of the cycle with the Cup and Hand Wash, so you're not eyeing off the dish soap at eleven at night.
Cup or undies
Most people end up with both. Cup for the long day, undies underneath for backup, undies alone for the days you can't be bothered with anything internal.