What's a 50-50 Grind? A Grind is the name for sliding along an edge (such as a curb, bench, rail, coping, etc.) using your trucks instead of your wheels or deck (read more on what a grind is). A 50-50 Grind is where the edge or rail being ground is in the middle of the hangers on the trucks. The name "50-50" refers to being half on the edge and half off, with both trucks even.
You will need to know how to Ollie before learning how to 50-50 grind. Read How to Ollie, and get comfortable with your Ollies first. You will need to be good enough to land your Ollies flatly, and you will need to be able to land with your feet where you want them on your skateboard. If you are brand new to skateboarding, start with the basics (read Just Starting Out Skateboarding).
Make sure you read all of these instructions before you try to 50-50 Grind. Once you feel good and are ready, go for it!