Money struggles

[talisman]

Which facilities are most important?

All of them, for varying reasons.

Medical gives you Red packs and reduces the time for recovery from injury - you will get an extra Red pack if you don't get any bookings in a match you attend.
Training gives you Green packs - if you are present at the match always do one substitution - this will give you one more Green pack.
Commercial gives you extra money on your home match day, the amount varies depending on your match attendance.
Improve your pitch to reduce the chances of injuries happening.
Add extra seats, update the stadium to give you bigger attendances and therefore more money.
Improve parking as this facilitates the attendance increases for your larger capacity stadium.
Maximise the revenue from your matchday attendance by increasing your ticket price by one each match until attendance starts to drop off then go back one.
The Youth setup gives you young players occasionally, useful in the early seasons when they will be of a quality similar to the main squad, but once you have mostly 5*/6*, the 'free' one will be useful for signing and selling to raise funds.
Stadium lights and scoreboard don't appear to affect anything but you will need to upgrade them eventually to do stadium upgrades.

[lastman]

All of them, for varying reasons.

Medical gives you Red packs and reduces the time for recovery from injury - you will get an extra Red pack if you don't get any bookings in a match you attend.

You get the packs, the rest is just hooey...

Improve your pitch to reduce the chances of injuries happening.

You'll have to upgrade at some point, but it doesn't do anything at all; same as above - just cosmetic...

Maximise the revenue from your matchday attendance by increasing your ticket price by one each match until attendance starts to drop off then go back one.

Very important, this one. Since you're dealing with little margins all over the place, you won't want to miss this: Always go for a sell-out rather than the maximum possible income here since it'll give you the few extra % in possession which in turn increases your chance of winning.

The Youth setup gives you young players occasionally, useful in the early seasons when they will be of a quality similar to the main squad, but once you have mostly 5*/6*, the 'free' one will be useful for signing and selling to raise funds.

Ought to be one of your first builds/upgrades.

When planning the upgrades, try to make sure not to waste any days without building anything. Also try to schedule the upgrades so that you won't interrupt 'pay-out' cycles, i.e. start the upgrade of the medical facilities on the day you receive red packs, training facilities on the day you receive green packs, youth on the day you receive youth players, and you're good to go