Is a sump necessary?, or is it mainly to keep the equipment out of the display tank?
If its necessary, I need some help on setting it up, what size I need, etc. If not, how should I set up my new marine tank?
any help is appreciated.
Depends on what kind of tank you're wanting to keep. A sump has not only the benefit of hiding equipment and culturing future food sources for the main tank but it also provides more water for the entire system. If you have a 55 gallon main tank and a 30 gallon sump, you really have an 85 gallon setup which, theoretically, should have more stable water parameters than a 55 gallon.
Sumps definitely can be advantageous but you don't absolutely have to have one to setup a saltwater fish tank.