Most people suggest buying things. I would slow that down.
1) Game Shape is a major part of the game. Make sure to keep your players 7+ GS when possible. 48-75 is the large window to shoot for on all players, with a refined window of 50-65. The closer i am to 55-60 each week, the closer my players are to 9 GS. Players at 9 gameshape will play above some of their ability because they are managed well, whereas a 5 gameshape will make their performance suffer even if their skillset is good.
2) Take it Easy is the go to for games you know you win or lose without much thought. Saving gameshape will promote your teams effort on normal effort. a team with 5 enthusiasm using a normal will lose 10-20 points all other things equal on a team that has TIE'd a bunch of games until they got to 10 enthusiasm and used a normal in that game. If you don't understand from this post, there should be plenty of information on it, but this is a major stumbling block for many players who cant break through, because people who do this well move up.
3) with your specific question, i would invest in arena. get bleachers to about 7k, 750 into LT, 200 into court-side, and 10 into luxury boxes sooner rather than later. You'll want to get to 12-14k arena size pretty quickly, but you'll lose out on significant revenue if you don't upgrade your arena to the point where it finally doesn't sell out. Each seat above being sold out is lost revenue for you, which means less money going forward. Keep 100k behind for player investment, and sink everything else into it for now.
These are the 3 most important things. I'd advise
against training for now, get to learn the basics above first. Training is very hard to master, and it can be a bit too much to learn everything at the same time. Also, training costs money while not providing any money for quite some time (unless you do it well, which as I said can be hard for a new player and even then you need to substract cost of trainer). It might sound too straightforward, but money makes BB go round so you need to spend it wisely as you can only spend it once. I think it's best to spend it only if you can use it effectively, so unless you have a good idea how to train, don't spend money on training. If you don't know how to keep your players in shape, don't spend money on players.
That said, the fact that you used to forums to learn about the game tells me you won't take long to master the 3 points above, so you'll get to spending your money soon enough ;)
I'd say before you start training, try to learn how to get players 48 minutes in a game. If you can't, you won't be able to train 3 players in 1 position or 6 players in 2 positions. Also it can help if you better know how to win games before you start training, because doing both is harder than each apart.
Another tip is try to evaluate your chances against other teams, because if you know your chances you can adjust your strategy accordingly. You can decide wether you can afford to play Take it Easy (TIE) or play your starters from the bench so they won't make too many minutes. As you can see, this skill is important for the 2 points above. A good starting point is using the website buzzer-manager.com, go to your roster page and check the total DMI, then go to analyze your next opponent and do the same. If there's a bigger difference than 20% between the two values, there's a good chance the higher DMI team will win. So if your opponent has 25% higher DMI, it might be good to just TIE so you have a better chance next game.