Decide whether you want an immigrant or non-immigrant visa. Immigrant visas include family related visas, permanent work visas, asylum visas and more. Non-immigrant visas include student, temporary worker and other types of visas. See uscis.gov for the complete list.
Submit an application for the visa in which you qualify. You can apply online at uscis.gov after creating an account. You can save information as you go, but the first step is choosing the visa for which you want to apply. Complete the steps as directed and provide additional information as required. Submit your application and any payment of fees when you're prompted.
Follow all instructions sent to you by USCIS. Failure to do so may result in the dismissal of your application. You can follow the progress of your application at uscis.gov. When USCIS grants your visa, they'll inform you on how to obtain it. You may need to provide supplemental information, be fingerprinted, go for an interview or send your passport to have the visa entered.