Sorry, I have no recommendations for sites to get you started. Probably local home centers have DIY books to offer ideas.

My suggestion is to first establish how you will use the space and the corresponding finishing touches; e.g. extension of the house. children's play area, apartment, entertainment space, etc.. Then draw a layout - measure, locate fixed entities, position approximate walls. Graph paper and a pencil will suffice.

Our basement is conventional, We actually placed masking tape on the floor to "live" the space before beginning the build-out. Lighting, receptacles' and built-ins were positioned accordingly.

This is not a simple matter because you will need to "shop" in order to obtain sizes of various of desired fixtures to make certain they will fit. I gained a new respect for architects while doing this. But, our final build has modest variations to the plan. And, should you hire a designer, providing your insight will aid them to deliver what you want. For certain, a back-up sump pump, humidity control and depending on your zone the possibility of auxiliary heat.

I've linked our finished space with explanations accompanying many. It was entirely DIY, done over the course of five winters. Good luck.

Here is a link that might be useful: Basement

Before you finish your basement, you need to be sure the basics are right. The best site for accurate information is Building Science Corp.

Don't assume the builder has got it right!