You set up your password when you sign up for online services. You then have to wait for an activation code to be sent in the post. The activation code is the series of letters and numbers beneath a removable flap in the letter that HMRC send to you.
I've set out below what you need to do to get a new password.
and follow the instructions. Select "Individual" and then "Self-Assessment" in Section, 1. In Section 2, choose whether you want to receive the new password online or via the post and complete Section 3. If you enter the correct information, you will get a new password by the method you chose, ie by email or post. Clearly, email will be the quickest method.
If you do not have all the information you need to get a new password using the automated system, then I'm afraid you will be left with two choices as follows:
1 If you need a new user ID, complete the form here
and submit it to HMRC electronically. A new login ID will be sent by post and you will be given instructions to set a new password or
2 Contact the online services helpdesk by calling the number here
and speak to somebody who will help you.
Let me know how you get on.