I see that your new to the game, welcome to MFN.
Yes, we are the GM's and HC for our teams. You make all the roster decisions as well as maintaining the salary cap and the game planning for each game. There are some leagues that are "Roster Only" leagues where you only GM the team and don't coach if that is for you.
There's a bit to learn about this game. I see your first league is one of the Public leagues. There are a few good Public leagues there. There are also some good, active and established Private leagues out there with good commissioners that can help you learn MFN.
If your interested in that route, my suggestion would be to pm the commish of the league your interested in *before* joining it. If the commish doesn't respond to you or responds in a negative way, move on to the next one til you find the league that suits you.
Continue to read the general forums. There is some great info in there. A lil complaining here and there as with any game. In my opinion most complaining shows the passion GM's have for this game and putting stuff out there helps improve the game. But for the most part some great info. Maybe read the forums from league MFN-1 as well. That is a beta league with some good info in it.
Good luck and enjoy this football world.