As you level up and see the increased yardage, the driver is the first thing you should concentrate on. Getting as close to the green and using the higher lofted clubs on your approach shots is key to leaving yourself short putts and birdie chances.
Second, I would say a putter in order to get more consistent on the greens, also very important.
After that, the wedges are key and upgrading irons when you get to the really long tees and have more and more long approach shots.
You can earn credits using the free offers, and finance most of this - but you fill find a day when you just don't want to wait, and buying credits/clubs just seems easier, lol.