RV's can be tricky business, start off by matching your orbit with the target's, a little higher (and slower if target's position is behind your vehicle) or a little lower; the higher the height difference the higher the approch speed. when your closest approach is at a reasonable distance (it may take a few orbits; I usually go for less than 1km separation) start maneuvering to match speed with target and finish the approach using RCS. Also, remember to right click on the docking port of your ship and select "control from here" to maneuver more precisely. There are a lots of tutorials on the internet too, check out Scott Manley's on YT