If this is your first time making one - Which I'm sure it is. You might want to read up on some tutorials first before jumping in.. Well I did but that's not the point.
When I started off making graphics I simply messed around alot, it could help you alot, tutorials are always a good-go aswell, but keep in mind not to copy them; You won't actually LEARN anything like that, use the information you've gotten from the tutorials and use that in future works, that's how learn ^^