I love the loaded up and release of a greenough but it is a more advanced skill.
Have a read of the the Fin primer to get an understanding of how fins work and the features you might like at this stage.
Consider too what you want to do with your surfing some fins will engage for that.
I would suggest you have a look at the Futures Fins RA ( Robert August) fin or their cutaway both at 7.5" that would be good starting point.
Another way to go is attend a swap meet where fins are swap or can be bought.
Depending on where you live there are other options and types.
A good thing to consider too what sort of wave you are going to be riding in the main.
get a set of different fins , you don't play golf with one club!
Having said that I will now say the opposite, get one favourite fin and learn all its characteristics and they effect your surfing , it will give you a lot of insight as to how they work!
A TIP. When you are up and riding and nothing is critical have a look back at what your tails is doing , push some small turns, observe and understand what is happening back there.
Watch other boards and what the rails and fins do a the surfing happens.
emulate successful things rid yourself of unsuccessfu things.
Mainly just enjoy

I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷