Depends on your budget, I personally use Autodesk inventor because that is what i learned with through school in addition to getting a free copy through a competition. But if i had to pay for a cad software, I wouldn't actually buy it. Another option if you plan on spending money would be SolidWorks. just google both of them and you will find that their is bigger preference towards solidworks in the proffessional setting. But, I have also found that most students learn with Inventor.
Now... If you dont plan on spending money, google has a basic cad software called sketch-up that is really popular among hobby robotics. Their are boatloads of tutorials out on the web for all of these software options, along with I think that all three of them come with tutorials when you download the software.
For now, just go with sketch-up as it is free.
hope it helps,