Help for newbie to make meshes solid

Hello everyone. So, I am totally new to Blender and just getting started with the course. I do have some work to get done and would appreciate any help from others. As seen in the uploaded photo, I have these models to work from a CG artist that aren’t watertight for me to 3D print them. I’m getting them hollow and really don’t know how to make them solid. I’ve figured out about subdivisions to make the surfaces smoother, but my 3D printer is having a bit of a struggle trying to repair the mesh. Would be so much better if I could do the work before I send it to the printer.

Thanks for any guidance.