The game's live now, so you can pick your team any time between now and the 15th of August.
You get a £100 M budget to select 15 players to include a sub goalie and three other subs. If you're not really sure where to start then you can get the game to auto pick a team for you.
Generally there is one 'gameweek' per round of Premiership games, and you get one free transfer per week to modify your team. You can make more than one transfer per week, but it starts to cost you points. You also get a wildcard, which can be played in one game week, which gives you unlimited free transfers for that week. Most people tend to use it either a few weeks into the season as they see who the form players are, or in February after the transfer window closes to take advantage of any new players.
Oh, and it's completely free to join and play.