Sometimes what I think might be an intuitive function turns out to be not as obvious to many computer users. One customer I worked with today hoped to create an online portfolio to track stocks. After arriving at the site, no matter what username and password combination she tried, it wouldn't work. When I asked if she had registered or signed up with the site, she responded "no".
After looking around at many financial and other other sites, the username and password boxes are prominent and the sign up or register links aren't nearly as obvious. A user is easily lured into thinking that by simply thinking of a username or password and clicking sign in. However, you always need to sign up or register with the site first. Think of a unique username (some sites require you to use your email address as your username) and a password (try not to use the same password on all sites). Then click the register or sign up link and fill out the required form. Some sites require an additional verification step where the site will send you an email with a link that you click to verify your registration.
Once those steps are finished, you can now log in and use the site you signed up to use. Be sure to keep track of your usernames and passwords in a central place so that when you need them, you can find them.