Get Buddies
Make sure as soon as you start unlocking buddies you don't do a level without them. Buddies are an invaluable tool that are worth paying out the Rings to get as they will help you achieve your goals.
Follow the Rings
Try to follow the paths of the rings closely as doing so will almost aways take you over obstacles and enemies and guide you to foes you can jump on to bypass obstacles. Learning to follow the path of Rings and trusting where they take you will come naturally to you the more you play the game.
Hitting Obstacles
If you hit an obstacle and you lose your speed you will find getting back on track difficult as you will not have the velocity to jump clear of enemies. If this happens the first thing you need to do is make sure you have enough rings to withstand being hit again so you can continue which means quickly moving on and grabbing the first rings you see.
Lower Path
You will find that the lower path will generally be the safer and easier option than the higher path which tends to include more enemies and have trickier jumps.
Character Upgrades
When you are struggling to progess don't be afraid to go back and complete objectives and collect Rings so you can spend them on character upgrades that will give you the skills to make later levels easier.