I have an Elevation 3/4 Size Acoustic Guitar, as a normal size one is too big for me right now as I'm only 14 and it's my first guitar and money is tight. When I tighten the strings to tune it, the low E string makes a pop sound and goes really loose. I looked at the place where the end of the string sits at the top and saw when it makes the pop sound the tube thing that holds the string jerks back to its original place. I'm just wondering what is doing this and how to fix it? Any help is really appreciated.
The screw at the end of the tuning machine can back out and be loose, causing some free play between the gear and the housing, causing slipping when tension is applied.
Try loosening the string until there is no tension on the tuner and then gently tightening the screw at the gear on the peg that is slipping. You may have to wiggle or pull in the post towards the gear to get it to seat.
If the screw does move, continue to tighten it until it seats and then add a little more tightness. Don't over-tighten the screw, if it backs out with use you can tighten it a little more next time.
Sometimes you have to remove the screw and gear and pull the post in towards the tuner before replacing the gear and screw to get it to seat properly.
If the screw is not loose and there is no free play of the plastic string post then it is possible that the tuning machine is faulty. In some cases I have fixed a gear slipping problem by exchanging the gears between two different posts.
This video looks like a good guide. Please try installing the strings according to the instructions here and update the question if you still have trouble.
If this does not work for you, it's possible that you have a problem with the actual tuning machine, which is plausible based on this from your question:
the tube thing that holds the string jerks back to its original place
In that case, it would be worth taking the guitar to a local shop and asking for advice. Most guitar shops are willing to look at issues like this and give good advice.
EDIT: Note that the "handy string winder" is a convenience only - you don't need the electrical one, but the ordinary hand-powered one can be a help.