My first 3D printing - I need some advice or tips
I see this:


  I had to make a small prototype and it requires an enclosure


and this:


  yes, it is hollow inside


How do you intend to get your item inside the result? This part really needs to be two pieces with two separate prints. Basically, a box and the lid, where you leave space for screws, include tabs, or plan for glue connections.

Finish that alteration, and the support issue suddenly becomes much easier. Most slicers have options to generate support for you, including limiting support to spaces between the model and build plate. Now you can rotate the angle at the which the part is printed in order to minimize your support needs, or so that most of the connection points for the support will not be visible in the finished product.