Hi! Although I'm no expert, I had a lot of issues with humidity when I started out as well, and had to do a lot of reading/question asking on the topic.

First, what kind of lid do you have? Different lids may help or harm the humidity levels in your tank, so you may want to experiment with lids. Mine was a mesh lid, which holds in approximately no humidity whatsoever, so I ended up laying a sheet of plastic with holes cut in it over the lid to preserve some humidity. It's rather unsightly, but it's a good temporary solution.

Some bad advice I once received from a pet store employee was to keep a wet sponge in my terrarium to maintain humidity. I had it in there for a few days before being told that it's practically a breeding ground for bacteria, so... don't do that, lol

A great way of maintaining humidity is to buy a spray bottle and mist the cool side of the tank every once in a while.