You have some great advice from those above, but being relatively new compared to these long timers here is how I incorporated various ideas.
I saved credits by using the "rock" or unlimited play ball. I played mostly on the tablet where I could "win" will eventually become worthless coins. The coins I won eventually allowed me to use a couple free coin rentals for a week of slightly better clubs. These clubs helped me just enough to manage on the longer Pro tee boxes.
The reason I ended up on the longer tee boxes so quickly, I used the free rentals offered by WGT while playing ranked rounds and shot better scores. Doing so moved me up the tiers where I was stuck with starter equipment. Hence the frustration. You can use the free rentals in coin matches and it should effect your advancement.
However, after advancing to Pro and Tour Pro I needed more distance than the standard clubs could provide. I was holding out for the better equipment upgrades in L50 and above.
Lots will advocate getting the L48 TM driver, but here is what I found. Since I was using the starter balls, the meter was rather quick. My drives were erratic, so I passed on it. I waited until L55 for my current driver and I do not regret it in the least! I believe it is a better driver all around and worth waiting a few more levels. I may even be able to hold onto it until the top tier drivers come available in the upper L90's.
There are some good 3 wd options out there. Again, I waited a little longer than some and I got one that was more forgiving and provided good distance. I have only recently upgraded from it to a slightly longer version of the same model.
I too played the L59 Ping irons until I could upgrade to my current set.
Wedges. I went the affordable route of the AVG. I simply couldn't justify spending nearly an extra 1000 credits to round out my bag with Clevelands when 3 reasonable performance wedges could be obtained for the price of one Cleveland.
By waiting to upgrade you will also learn more about course management and how to get up and down to save par, a crucial skill you want to develop. You could upgrade these wedges as soon as they are available to you for the discounted price.
Putter can be at your discretion. Some feel the putter is the best club in the bag for starter gear. Also, it won't matter how much you spend on a putter if you cant read putts or get a feel for distance and speed. Save your credits until you feel confident in these. After that it is a matter of fine tuning what you already learned.
Beyond that, read the forums, watch videos, and practice. I have been playing for just under four months and I feel I am doing rather well. I have leveled up by shooting good scores rather than using the WGT glitch of beating higher tiered players in match play. Hence, be careful playing match play against Masters, Legends, Champions and winning because it will artificially lower your score much more than it should otherwise.
I know some will say good luck beating a higher tiered player, but I have also heard stories of how someone got screwed over by a higher tier player that threw the match, lost their connection and forfeited or whatever. Either way, it is a win and even if you were only playing par golf or made one birdie it could lower your scoring average as if you had shot a 53. It is crazy.
Best of luck and invest wisely!