Write a killer script. Sit down and figure out what story you hold inside of you that only you could tell and write a script that nobody could refuse.
Read Variety magazine and look for stories about screenwriters or recent scripts sold to see who is in the market for buying a script and who isn't. Find an agent on an online listing of agents or through a publication such as Variety.
Make a list of 5 agents to send your script to. Send your script to an agent, or drive it to their home and drop it on their doorstep if necessary. Be persistent and don't take one refusal as an indication of your talent.
Make sure your agent isn't pulling the wool over your eyes. Be persistent in contacting your agent. Ask your agent daily where you are on your script, who's reading it or thinking of optioning it.
Discuss a reasonable amount that your script would go for and ensure that you are adequately compensated for your script. Read the contract very carefully and make sure you will be billed on the screen and will receive some of the box office royalties.