Just wondering what the coins are for and how to use them?
You use them to ultra evolve your monster but if you just started you don’t need to worry about it now
Used for feeding your monsters de-aging (youth) fruits as well.
How do you buy fruits?
You don’t. They sometimes drop in Online Missions or you can go to the actual youth fruit mission under: Online -> Ultra evo & events -> Youth fruit
Considering that you’re still new to the game you might not be able to access it yet. The game will tell you how to get these.
Ok thank you 
also is there a way to merge to monsters together when there are duplicates rather than just deleting them?
Ok thank you also is there a way to merge to monsters together when there are duplicates rather than just deleting them?
Nope the merging of duplicates only happens to duplicates you roll in the eggs and monsters from live events such as the current Trial of Light event. Monsters you catch in the wild cannot be merged, only deleted.