log in
No object
Enter a computer account
Log in with your email address and password.
Usage
Also 'log on'. 'Log in to the site', never 'log in the site'.
No object
Nothing follows the particle directly. You cannot say it with a direct object.
Practise it
Add it to your daily practice or see every verb with up.
Further reading: log in on Wiktionary (CC BY-SA).