If you're new to 3D modeling do the following before starting.
1. Download 123D from here http://www.123dapp.com
2. While that downloads watch these tutorials http://www.123dapp.com/howto/design
3. Open 123D Design, play my video and have a blast!
Feel free to add changes at the end if you'd like, as you can tell I added a bottle opener/belt cutter to mine. The possibilities are endless thats what's great about 123D Design!
If you have any questions just ask! (: