There are a lot of ways you can get onto a 50-50 I will try to explain them all I have 3 ways:
1. Ride on to a 50-50

This has no real challenge to it at all. Although this is a good way to learn to grind but rarely will you find a grinding pole that you just ride on to. Just keep your feet over your trucks and it should be just like riding on the ground. If finding a curb that you can ride onto be prepared to slow down quickly.

2. Ollie Forward onto a 50-50

This is easier if you know how to Ollie high and have good balance when you land. Approach your target at a comfortable speed and Ollie. When you start coming down on to your grind then your feet should be over the trucks. This will take a while to learn but if you can Ollie high it shouldn't be a problem!

3. Ollie sideways onto a 50-50

This being to most popular way to get onto a 50-50 it is also the hardest to learn for most. First you need to learn how to Ollie sideways. Place your feet in Ollie position and when dragging your foot try to drag your foot out toward your target. Helpful hint: when Ollieing try to jump toward your target and keep the board directly under your feet. Try to keep your board parallel to your grind and your landing should be no problem. Practice this on a waxed curb or a fun-box if you have one!

Good Luck!