If you are just looking for a program to create beautiful shapes and designs which have barely non-existent mechanical requirements you should imho avoid programs like autocad, catia, ideas, solidworks, or even maya, 3dmax or so because you will need a lot of skill and time to create anything that looks like anything you wanted in the first place.
Imho, start with a painting program like gimp, paint shop pro or even photoshop and start creating simple contours and lines and try to create your designs that way. Of course it is possible to create breathtakingly good looking designs with catia, for example, from what I've seen, but it takes bucketloads of skill. Learning to do that by yourself just isn't going to be fun. Engineering programs are great fun if you want to play with some ideas and know how to get them done quickly - it's sometimes just fun to throw some pieces together just for the hell of it. But essentially that is just toying with the program and it gets clear very quickly that toying isn't what these programs are made for.
If you really want to get into some engineering program, buy a book which has examples. Books might be a bit expensive so you should invest some time into comparing the programs and understand what you are going to do with it. From my personal experience, books are the best source for information for learning how to use some engineering program.