Some very common tips ...
Resetting the root password
If you have forgotten your root password, it is pretty difficult to recover it from within the installed system, but you can use a bootable linux cd or floppy to set it to blank.
Note - the cd/floppy's kernel should have support for components like your processor, hard drive and filesystem being used.
Basic instructions - Boot into your cd/floppy, and mount your linux / partition (or /etc if you created a seperate partition for that) at a partition point like /mnt. Edit the /mnt/etc/passwd file, and blank out the encrypted password in the line for the root user.
Detailed instructions - After booting the cd/floppy, mount your linux partition usng the command
Replace /dev/hda1 with the linux partition.
Incase this gives a problem recognising the filesystem, use the -t switch to specify the fs type, like ext2 or resierfs. Also make sure the directory /mnt exists, else create it.
Edit the file /mnt/etc/passwd with vi or any other editor
blank out the password field (it will possibly be marked with x), such that the line looks like
root::0:0:root:/:/bin/bash
Note that the two colons after the first word "root" indicates a blanked out password.
Reboot into linux, login as root, you wont be asked a password. Use the passwd command to set a new password.