Per the message when you created your account:
"Take Note of your Private Login Key!
Below this you will find your thirteen word "Private Login Key". Keeping this secure and private is very important, as it is the only way that you will be able to login to your MyMonero account. As we don't store your private login key on the server there is no way to recover it if it is lost! Your private login key can also never be changed, and if it is stolen or otherwise compromised, you will have to move your funds to a new account with a new private login key. Therefore, it is best that you backup your private login key by writing it down, perhaps obscuring it as part of a poem or letter, and storing it in multiple safe and secure places."
You have two options in logging into your MyMonero Account
Option 1: Login with your Private Login Key which is your 13 worded password.
Option 2: Login with your Public Key which includes a. Your Monero Address b. Your View key c. Your Spend key. - These are found under your personal account details in your MyMonero wallet.
If you only have 12 of the 13 words of your MyMonero Private Key:
The MyMonero wallet uses 13 words as the private key the 13th word is the sum of all.
Therefore it is the same word as one of the 12 earlier words.
Simply try 12 combinations and you should come right.