Contact whomever you wish to license your music to, and have a pitch ready; this is known as "direct licensing." Direct licensing commonly involves a one-on-one communication process between the artist and end user, who could be anyone from a filmmaker to a marketing agency. Licensing your music directly to the end user allows you to avoid having to pay percentages to an agent, meaning you get to keep the money that you rightfully deserve. While more visible musicians will often be approached by those who are interested in licensing their music, those who are just starting off usually benefit from sending in a pitch.
Contact a music library with the intention of entering into what is known as a "buyout library deal." This is a fairly common music-licensing scenario in which you grant exclusive rights to your music and are paid up front with a lump sum. While royalties in this scenario are rather small, you can usually expect approximately $300--500 per song when entering into a buyout deal. Some musicians are contacted by music libraries, while others have to actively shop their music around to prospective buyers.
Enter into what is known as a "non-exclusive library deal" with a music library. While this scenario typically offers no payment up front, it differs from a buyout deal in that it can bring in a large amount of royalties over time. This type of licensing is especially effective if you believe that your music will have lasting appeal and will benefit from consistent replay value.