Hey Dude
the best way to start is to get a demo or educational cad application, solidworks is a good one and Rhinoceros 3d , but there are many out there,
go onto youtube there are heaps of tutorial videos showing from scratch.
The key to good design is simplicity , in geometry and creation , it may sound crazy but you dont want to get into a bad habit of messy creations,
learn how to draw a line or a circle or a rectangle, it is also important to learn the terminology used to create geometry , otherwise you can get a little lost if you don't know what a fillet is, other than what you eat!!
next is to think of a part you want to design and just play until you get it right, next thing you know you'll be into it
cheers