Sketchup is a great choice if you're new to 3D modeling: it's free, easy to learn and there are TONS of free tutorials available. Even so, you may need to know a few things before you can get your models 3D printed. 

Community member Marcus Ritland from Denali 3D Design shares 8 great tips on MasterSketchup.com. These tips include making your models printable, making them look great and keeping them affordable.

Don't forget to give him some kudos on the forum!