Hello and welcome!
1. Don't touch your arena just yet, and don't be too worried about buying players right off. Take some time and get accustomed to what skills are important to each position, and who works the best where on the team you already have. There are several newbie tips in the sticky thread in this forum
(3944.1), which is worth reading a few times.
2. Don't worry about changing staff until you actually need to do so. Since there are only a couple more scouting weeks in this season, I wouldn't invest in scouting until around the end of the season, when scouting investments for next season begin.
3. This depends on who you ask. About 12-14 players is the norm for most teams. Plus, you'll be getting 3 new players in the draft soon, anyway.
:)