So, you have Steam Login dialog in front of you, but you can't recall login (or password), while storing it in some safe place for some reason wasn't an option. Press I can't sign in button:


You will see a page where Steam will propose to enter anything you remember - login, email (which was used to register in Steam) or telephone number (yeah, you can bind phone number to your account, Steam used to mention it from time to time):


If you remember your email (and able to open it, of course), than problem's solved - instructions about password changing will be sended right to your email.
If you don't remember your email or lost an access to it, than type in your login. If Steam finds it, you will see this dialog:


Here Steam gives you a hint - two letters of your email. If you still can't recall it or don't know email password (and haven't bound reserve email to it, so you could restore access), than choose second option. There you will get huge instruction, so just follow it.

Good luck, don't lose your passwords!