The first things most people will want to do is get their hands dirty with the back-end. If you want to do any sort of content editing, or administration on your website, then you will need to access the Umbraco login page in order to access the site builder for your site.
To access the Umbraco login, you need to add on /Umbraco to your websites Url, like so:
When you access the Umbraco login page, you will be asked login (depending on which version you are using it will vary. this one is version 6, but, 7 looks more modern):
Enter your Umbraco username (in 7 this is your email address registered against your Umbraco account) and your password (the one you wrote down during the installation section) then press login. If you have forgotten your password I recommend reading Reset Your Umbraco 7 Password Manually via the database