If you want to work temporarily and already have a job arranged, the visa you need will depend upon the type of work you are doing, where you come from etc. If you click here and go to the list on the left of the page, you should find your category; if not, try the More Information section on the right or go to the Contact Us tab at the bottom.
If you want to work permanently, you should click here and otherwise follow the same procedure.
Generally, it is not easy to get a permit to work in the US unless you have already fixed a job with a legal employer. In this case, it will speed things up a lot if you talk to your employer first. If you are hoping to apply for a visa without having a job, it would be advisable to do some research first into how much work there is available in your particular field and how likely you are to get a permit.