How do you bevel wood edges?

Depends on the nature of the bevel, if you're attempting to match an existing profile pattern such as a "roman ogee" then you're into using a router. Based on how you've phrased the question (not knowing what the relevant tools are) it sounds like you aren't ready for doing that yourself. No disrespect meant there - just that routers are straight up dangerous and need to be approached with caution for any beginner.

If it's a plain 45 degrees then a good old fashioned hand block plane will do the trick.

If it's for a single radius curve then a router would still be preferable but you could do it with a block plane and sandpaper if you take your time.

A completely different approach would be to find a matching profile length of beading at a hardware store, which you could then use to edge band the shelf with. This brings into question the finish of the overall piece because you'd want the edge banding to match nicely.