You can always login through the following link: https://login.salesforce.com or through your org specific link. The org specific link is usually formatted such as: companyname.my.salesforce.com.
When you first sign up for your org, your administrator should have sent you a link to login. This link would be system generated and your admin cannot see your actual password. This email commonly gets stuck in your junk folder which can also happen with the forgot password link as well, so always check there for the login email if you are new.
Common Login Challenges
So that looks super easy, right? Well sure, but as you use force.com in more ways, you will almost certainly end up with at least a test box and possibly others to login to as well.
Just make sure you have the correct link and login for the correct box. For instance, sandboxes are made with new names and although you can use the same email in various instances of the platform, your login must be unique. This means if your username was firstname.lastname@example.org it may be email@example.com when you login to test.salesforce.com. Here is how salesforce support explains it here.
If you keep running into challenges, do not beat your head against the wall, just contact your administrator and they can usually help you out which may include resetting your password and sending you a new login link.