Finding Upwork agency

Hello all. I am looking for some resources on where to find agencies I can work for. I am mainly full stack web developer. Main reason is I don't know how to "sell" myself and write a good cover letter. I do believe I have great skills when it comes to programming with 9 yrs of experience. If I understood correctly, an agency would be able to help me with the problem.

Note: I read the rule no. 3 of the subreddit and I don't think this falls under "finding jobs", as I am looking for general guidance and resources. But if mods think I broke the rule I apologize and the post should be removed.