I still have decent side hair but to have the thick hair on the top that I would like I think I might need a full cap. I am 23 years old,so really thick look woudnt be bad. im thinking like 75% + density range but my side hair is really fine. Is a full cap alot more work and harder to hide ? Is it worth it or should I just start with a topper for now ?

I'd start with a topper if your sides are good. Plenty of time for a full cap later on. Both have their pros and cons. I started out with toppers, but now I'm a full cap convert.

Start with a topper. If your sides get too thin or the blending, color match, etc. becomes too much of a hassle, try a full cap.

A full cap is not much more work and it's not harder to hide since you don't have to worry about matching your existing hair.