im choosing a startup disk and its asking me to unlock it. is this normal and if i do it will it delete my files on the computer?

If you are being asked to unlock the startup disk, then it is most likely been encrypted with FileVault. You would not be able to use it unless you enter the password that was used to encrypt it originally. Unlocking it with the correct password will allow you to access its contents; it will NOT delete any files on it.

Then, I'm afraid that you will not be able to use it. FileVault 2 is a very secure means to prevent anyone from accessing the drive's contents. If you are not the original owner of this drive and did not encrypt it, there is nothing you can do with it expect repartition and reformat it. Any data on it will be lost.

If you encrypted the drive, did you create a Recovery Key or set if so your iCloud account could unlock it?

From the article posted above.

Choose how you want to be able to unlock your disk and reset your password, in case you ever forget your password:

• If you're using OS X Yosemite or later, you can choose to use your iCloud account to unlock your disk and reset your password.*
• If you're using OS X Mavericks, you can choose to store a FileVault recovery key with Apple by providing the questions and answers to three security questions. Choose answers that you're sure to remember.*
• If you don't want to use iCloud FileVault recovery, you can create a local recovery key. Keep the letters and numbers of the key somewhere safe—other than on your encrypted startup disk.

It shouldn't but you should backup your Mac on a timely basis regardless.

This may apply > Use FileVault to encrypt the startup disk on your Mac - Apple Support