There are three basic ways that you can choose to hold your acoustic guitar. The casual method, the classical method, and standing are the options that you have when it comes to holding your guitar. Try experimenting with each way of holding your acoustic and then choose the method that feels the best for you.
The casual method of holding the guitar is probably the most common. To use the casual method, place the guitar over your right leg and pull the guitar close to your body. Use your right arm to hold the guitar close to you and keep if from slipping downward. Some people like using straps while they are using this metheod. This just helps to keep the guitar stable and close to you. Now bring your left hand up and place it on the neck of the guitar. Make sure that both of your arms are relaxed and that you are sitting up straight.
To use the classical method of holding the guitar you will need a foot stool. You can pick up a foot stool at your local music store for about $15. If you already have a foot stool, place your left foot on it. Now put the guitar over your left leg. Put your right arm over the guitar to hold it in place. Now bring your left hand up to the neck. You will probably notice that the neck of the guitar sticks out to the left much more than it did when you were using the casual method. This is one thing about the classical method that is really good. Having the neck farther to the left gives you much better access to all of the frets. This method makes it easier to play more difficult things when they are higher up on the neck of the guitar.
Now let’s take a look at standing with your acoustic guitar. I like this method because if I am playing in front of people, odds are that I will be standing. Practicing like I play helps me to be more consistent. In order to stand with your guitar you will need a guitar strap. Most straps can cost anywhere between $5-$35. In order to used a strap on your acoustic guitar you will need to have a guitar with strap buttons, or you could use a strap that you can tie to the headstock of your guitar. Most straps are adjustable so that you can have your acoustic guitar in a high or low position. Playing your guitar in a high position gives you better access to the neck of the guitar and keeps you in a better overall position for playing. Playing your guitar in a low position looks cooler and it may be more comfortable for you. I recommend adjusting your strap to where your guitar is in the exact same position that it would be if you were sitting. This will make your practicing and playing more consistent.
Experiment with each of these ways of holding the acoustic guitar, and choose the way that works best for you.