How to design 4 stroke DOHC engine using SolidWorks?
If you're new to something it's difficult to know where to start. So here are a few pointers.
Draw each part as a standalone item, include only the essential, functional features.
Put the parts together in an assembly, applying appropriate mates.
Change each part so that they all fit together nicely (the assembly will update automatically).
Change each part to ensure they all work together properly.
Change each part to include the details you left off originally.
Change each part some more, just to make sure.
Then start again and do it the way you now realise you should have done it in the first place.
This might not be the best method but it works for me.
If you are new to Solidworks, first learn to use the software. As Manolis said, work through the tutorials under the help menu. They are excellent for new users. That will get you started. Then start modelling anything you can lay your hands on. Then contact your VAR and get yourself on the Essentials training. If you've invested in the software invest in your education in using it efficiently. Good luck.