The file is located in /etc/fstab. You will pretty much add an entry to it containing the same data as your manual mount statement. For example, to mount sda2 on /mnt/mydrive you would have an entry similar to this:
/dev/sda2 /mnt/mydrive/ ext3 defaults 0 0
I would recommend that you read the man pages (man fstab) for clarification on the meaning of the parameters for your mount as what I showed above is just an example.