Welcome to Upwork! I see that you recently joined Upwork, so you may want to check this video to help you get started with freelancing on Upwork.
On Upwork, you can search for jobs and submit proposals to jobs that fit your skillset and expertise. In the Cover Letter field, you can send a proposal to the client and let them know how you can help them with the project. You can also share some of your skills and experience that are relevant to their project. If your proposal is accepted you will receive a message from your client and a message room will be created where you can continue negotiating with the client. If the client decides to hire you, they will be able to send you an offer as well.
To find great tips for writing proposals please check these: