Always remember to keep on practicing and keep your instruments in tune especially when you are just starting out to play the guitar. If you have plans to tune your guitar using only your ears, then prepare to take on a difficult task. This skill takes a lot of time to master and perfect. You need to keep on practicing at it until such time you would be able to master that kind of skill. While you are trying to master such skill, use an acoustic guitar tuner first to tune your guitar so you can dish out wonderful music when playing.
You can find a variety of acoustic guitar tuners. One of the most common types of acoustic guitar tuner that you would encounter is the electronic acoustic guitar tuner. The needle of the electronic tuners indicates whether the string is either flat or sharp. The tune of the string is perfect if the needle is at dead center. The pitch is low if the needle leans to the left and high if it leans toward the other direction. To make sure that everything is in tune, a green light will flash if you play the scales. If the acoustic guitar tuner yields results that produce a red light, then the strings need some adjustments.
The other acoustic guitar tuner that has a built-in microphone is the acoustic guitar auto tuner. The string of a standard guitar is calibrated for this acoustic guitar tuner. You would know if you are in tune or not by just playing a note, no need for the scales.
The other type of acoustic guitar tuner is the clip-on tuner. There is no need for a microphone since all it needs is to pick up the vibrations of the guitar while being clipped on to it. It is very applicable when you are playing to a loud room and you can just leave it on the guitar in case you want to check if you are in tune from time to time.
You can find all these acoustic guitar tuners on your local music store but better ask the opinion of seasoned musicians on what they think are the best and most applicable to you.