Hello, and welcome. It’ll be helpful if you say roughly where you are, whatever you know about your trees, and so on. With that info people here will be able to help better.
A very general rule of thumb is to prune during dormancy to improve structure, meaning take out bigger stuff when the tree is dormant, and to prune during active growth to remove excess growth, open up the tree to light and air and such as that. But you can safely prune any time you see something that needs to go.
For starters: take out any dead, diseased, or damaged branches as soon as you see them. When branches are crossing take one of them out.