Money is important. You need money to upgrade and maintain all your facilities so having a higher income means you can have better facilities. Any money you have left can be used to buy better players.
Your stadium will be your main source of income, but you need to increase your fanbase. You can't start too soon on doing this. You'll get a bonus increase in your first three seasons to help you get started so make the most of that.
Always upgrade your fanshop to the maximum level. The fanshop can't have a higher level than the stadium, but (at least until level 6-8) it's definitely worth maximizing it.
Always upgrade your stadium as soon as you can. This is possible if the stadium is filled for 80% in 4 of your past 5 matches. Don't worry about not being able to fill it after upgrading, the possibility to upgrade the fanshop will make it worth it.
Watch your home matches while they are being played to get the 20% extra attendance if this can help you get the stadium upgrade sooner.
Sell tickets to your matches to maximize the gain in popularity.
