Another easy one to get, simply collect 50 rings - it can be done on the first stage.
If you are struggling then just take your time and try to avoid the badniks and traps. There is a save feature that you can use to save your progress at any point in the game so if there's a section that you think you're going to lose your rings by getting hit, save it and then attempt and if you fail, re-load the game save.