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.
4) Buy two guys to train. Single Position training positions for Guards are JR, OD, and typically PA too. Also having decent IS is desirable for many. Single Position training for bigs is IS/ID/REB, with many desiring at least 5-6 OD and PA. Single training = skills where you can only use 1 position to train at maximium efficiency. for guards increasing OD for example, the most efficient spot to train is at PG in single position. You lose training speed if you make it two position or shift it to SG, so it's not as worthwhile. Shoot for 7 potential players that are slightly behind at 18 (at the beginning of the season, this would be 41-44 TSP, while most start at 47-52. Right now, those 47-52's are 60-63's, and the ones behind are low 50's). Train them in 2 position training as well as a draft pick and maybe a few players you started with that are decent. I advise one on one @ forwards (SF/PF), and get them 48 minutes a week in those two combined (48 in SF 0 in PF, 33 in SF 22 in PF, 10 in SF 47 in PF, whatever it takes just make it total 48+).
Good Luck.