There are a few different ways that you can login to your Gmail account. The most common way is to go to the Gmail website and enter your email address and password into the login fields. If you are already logged into another Google account, such as YouTube or Google Drive, you can click the "Sign in with Google" button on the login page and enter your email address to be taken to the Gmail login page.
If you are having trouble logging in, you can click the "Need help?" link on the login page and follow the instructions to recover your account.