Getting the optimum amount of wraps around the tuner post when changing strings can be challenging for a beginner. There are many ways to string a guitar, and most of them valid. Try a few different ones to see which you like best.
Here's how I do it on a Fender-style guitar with slotted posts:
Step 1: Remove all of the old strings from the guitar.
Step 2: Beginning with the low E (6th) string, grab the string at the bridge and make sure it does not twist by sliding it through your fingers as you pull it towards the headstock.
Step 3: Place two fingers on top of the nut and pull the string snug.
Measure where to cut the string by stretching it to two pegs past the one you are stringing. For the low E string, you would use the D (4th) string's tuning post. Remove the two-finger bridge and grab the string as shown above.
Cut the string with a wire cutter. Planet Waves has a Pro Winder that is ideal for this job.
Step 4: Place the end of the string into the hole of the tuning post.
Step 5: With your right hand, keep the string taut while you wind the peg. You'll need some support for the neck. I'm using the Planet Waves Headstand.
Tune the string to pitch, making sure all wraps go under the previous wrap. This should give you 1.5 to 2 wraps on wound strings and 3-4 on plain strings.
Step 6: When you get to the B and E strings, measure by grabbing the string at the target post and pull it back by two pegs to measure, then cut.