Hi Chewy,
I'm pretty new, so my advice isn't going to be strategic - you've got a couple of big guns firing you advice on that front already. But there's more you need to know. You've arrived in the game in play-off time. That means you won't have many fixtures - and without playing games your players lose fitness (stamina) really fast. So you need to fix up scrimmages (that's US speak for friendly) - and you must do that before Wednesday if you want a game on Thursday night. So you'd best set out to challenge some folk. You've got an automatic scrimmage set for next week against the team who finished equally low to you in your league. But that's not enough. You need at least two games a week to give your players a run out.
One other thing you should check out is whether your team has got any info on the 'draft' - like all US sport, buzzerbeater has a system for pulling young players that happens once a season rather than regularly like European sport. Under the players menu there's a link to edit your preferences in the upcoming draft. If you've got any intel there at all it should help you choose which order you'd like to pick from the available youngsters. Since you've finished the season so low in your league you will get early picks in each round of the draft so this could help you get a couple of good players.
Finally, the thing I've found the most getting used to is the training regime. Players only train if they get sufficient court time, unless you pick one of the special 'team' training options, like training stamina. Younger players obviously train better than oldsters, so working out which of your youth are going to get most minutes on court and setting up training to maximise the impact on them is crucial. And difficult.
Good luck!