The general rule of thumb is to use lower case, and to use the conjoined version of a word (eg ‘website’ rather than ‘web site’):
Login vs log in
This one can be a bit tricky, the rule is to use the conjoined form when it is a noun or adjective and the separate form when it is a verb.
Enter your username to log in
Exceptions to the lower-case rule are brand names such as: