When it comes time to change your guitar strings, don't remove them all and then re-string the guitar from scratch. That just makes your job harder than it needs to be. It's best to remove and replace the strings one at a time, from the thickest string to the thinnest. If you remove all the strings, the tension in the instrument is lost and you will have a harder time getting it in tune, not to mention it'll be harder to wind the first couple of strings as you try to bring the tension back.

You also might want to take a look at the Jamorama Guitar Learning Kit -- It's a very popular multimedia guitar lesson package which includes the "Guitar Tuner Pro" Software and "Tuning Your Guitar" eBook: http://www.learningguitarlessons.com/products.html#tuner

That's something that's really helpful after changing your strings, so you can get them back in perfect tune.