Finding a helmet that fits you is the most important part. That will keep it on your head while you fly through the air or over the handlebars. Work on the straps to get the fit just right. It's easier if you have somebody help you.
You don't have to pay a lot for a good helmet, but be sure it fits you, you like it and you will wear it. Your helmet is good for only one bad crash. That damages the foam and you have to replace it. So you can wear a bike helmet to skate if you use inline skates and don't crash every day.
For skateboards where you do crash a lot, you need a different helmet that recovers after a crash. Skateboard helmets should have a sticker inside that says ASTM F-1492.