You are really making us work to answer your "quesion".
Let's start with "I've ridden them before, but nothing big" .
What's "them" and what's "nothing big"?
Check the right forum "What Bike Do I get" and read the "Hints" thread. Post your answers. If you don't give us any info, we have nothing to answer.
If you are at a complete loss, what to ride, how to ride, where to ride, what it costs, what to wear, "how to wheelie".... you need to read some of the 1 millon topics here that will give you all the answers you can consume. THEN ask specific questions.
Sorry, there is no 10 minute, 10 step guide to "You too can be a motocross rider".