Those who wish to enter and work in the United States must first obtain a work visa. If you want to work in the country temporarily as a non-immigrant, then under the US immigration law, you must apply for a non-immigrant working visa, which will be based on the type of work you will be doing. Usually, those who fall under temporary worker category are usually required to have an agent or employer file a petition or Form I-129 for them. But since employers don't usually offer such luxury as assisting you in your application, getting an agent is the best choice you can take.
If you want to permanently live and work in the US, you must file for an immigrant visa, which requires your agent or employer to get an approval from the Department of Labor. Once done, the agent or employer can file for Form I-140- Immigrant Petition for Alien Worker. The process of filing for petition is long and complicated. If you want to save yourself from a lot of trouble, it is better to look for professional and experienced help to do the procedure.
Once petition for each of the visas is approved, your agent or employer will receive Form I-797- Notice of Action. Then, you can finally apply for a work visa at the Embassy. Do not forget to bring your petition receipt during your interview as the consulate has to verify its approval through the Department of State's Petition Information Management Service (PIMS).
The requirements and process for getting a work visa depends on the type of work visa. There are several types of work visas for the United States: H1B, H1B1, H1C, H2A, H2B, H3, L1, O1, P1 and E3.