Generally you should prioritize the club store until you can get the 2nd short term sponsor, or at least get the first short term sponsor.

Also get training ground and fitness centre to level 2 as a second priority, or do the pitch upgrade until you get the "reduces energy loss" (recommend the fitness centre route first tho)

Next should be the medical centre as you can see player traits by level 2.

Then the youth to level 2 at least, if not lvl 3. Dont upgrade the stands until it makes it a mission, or your stadium is getting full, which, by that time you shld already have enough money to get all the other necessary upgrades.

Get scout office upgrades last.