How can I allow only a certain user to mount and unmount a drive ?

read man mount

If mount a device as root, can I unmount it as a normal user ?

No. A normal user cannot unmount a drive that root mounted. If it is a linux partition, you can make all directories and files on the drive only accessable by a certain user or group, but not if it is a FAT32 or NTFS one. Perhaps you can make another sub-directory under /mnt or /media (wherever you want to mount it) and set that directory's permissions. Like:

/mnt/user/partition

mount the drive in 'partition' (or whatever you want to call it)
give 'user' permissions so only a specific user or group can access the drive.

I don't know if this will work, but it is worth a try.

when the computer boots up, it mounts the drives using /etc/fstab, these drives will be root mounted, can I set an option to mount the drives by another user at startup ?

I don't know of any way to do it. But why would you need to? If you want a drive to be mounted at startup, why would you need to unmount it later?
If you want only a specific user/group to access/write to some files, make subfolder(s) and set the permissions appropritely. This also has the advantage of each user having their own files on one single partition.

You can put 'user' between the parameters in the specific fstab line. Like this:
That way a normal user can mount/umount partitions (or cd's and stuff). Also, when mounted at boot, a user should be able to umount it anyway. So I don't think mounting it as root will prohibit any user from unmounting it with the above option in the fstab - but I'm not 100% sure about that.