MILILANIDON: What should I be doing to build up enough credits to upgrade clubs and golf balls?
some good advice given above.... I will add is try to "limit" your upgrades otherwise the expense grows each time.
I started on the mobile platform and didn't upgrade any club until around L40 and into the 50's because while I was learning on the mobile platform playing coin games, I won enough coins to "rent" the better coin rental clubs to allow me to reach the higher levels so I was not upgrading my clubs twice… this saves credits, which you should still be trying to do slowly by the 2 credit videos.
This takes time, but it is possible... but if you don't want to spend actual money then you often have to grind for awhile...
Also, I would not invest in "expensive" ball upgrades in the lower levels because you will be bleeding credits far faster than you can replenish them through the 2 credit videos.
When you reach a point where playing a better ball may help, play the least expensive ball that gives you more playability without forcing you to spend hours replenish the credits for me it was the lower level Srixon balls when I finally started buying balls for credits. Also, take advantage of the occasional free ball offered just for signing up for the monthly tournament, yes it is only one ball but I still use those in coin games on mobile.
Also, on mobile you can slowly "upgrade" your equipment with the sponsor apparel, which can allow you to play for fun until you save up the credits.
Just a few other ideas for you...