Use the Right Pick
Pick choice is important for all guitar playing, but even more so when it comes to acoustic guitar. Since you're not plugged in, your pick is essentially your amplifier. Just like an amp, different picks will change the frequency EQ of your sound.
Thinner, flimsier picks will produce a bright sound with lots of mids and highs. These are ideal for songs that heavily rely on strumming open position chords. Thinner picks are also great for funk and reggae music, as they make fast strumming easier.
The flimsiness allows them to slightly bend as they rake across the strings. This leads to less resistance when strumming multiple strings at once. Strumming with a stiffer pick can lead to a slight scraping sound as you strum, due to the pick not bending with the strings.
Thicker, sturdier picks do have their place in playing songs with a lot of single note, straight picked lines. The stiffness allows for the precision needed in alternate picking at high speeds. Since the pick won't bend with the string, it becomes easier for the player to maintain a consistent picking motion without slipping.
Use the Right Strings
String choice is also important in getting the desired sound on acoustic guitar. Much like pick choice, it comes down to the type of music you'll be playing. If you primarily play strumming songs, light (.012's) to medium (.013's) gauged strings will produce a bright, lively sound. Too thick of strings for strumming can sound too dull.
Conversely, if you play a lot of alternate picking and riffs on the acoustic, you could go two ways. For bluegrass style music, extra light strings (.010's) can work well for the high speeds. However, extra light strings will produce a very thin tone. If you want a beefier, thicker tone on the acoustic, consider playing with heavier strings (.014's).
It is important to note that the higher gauge the string gets, the harder you have to press down on the fret. Some players avoid heavy strings simply because they can make the guitar harder to play. Once again, it comes down to preference.
Learn to Fingerpick
Fingerpicking is a staple of acoustic guitar playing, but can be tough to get the hang of. It can take several months for even advanced players to train their fingers to cooperate if they have never fingerpicked before, so don't worry if it takes a while to get going!
Growing the fingernails out on the picking hand can help significantly when fingerpicking. Not only does the fingernail produce a more desirable tone than a guitar pick, but the nails also make picking each string easier. This can be achieved with just slightly longer than normal nails, so you don't have to grow them out crazy long.
Fingerpicking involves the thumb, index, middle, and ring finger of the picking hand. The pinky is generally not utilized in traditional style. For most songs, the thumb will handle notes on the E, A, and D strings, while the other fingers tackle the higher G, B, and E strings.
To get started, here is a basic fingerpicking exercise. This figure uses an open position D major chord. The fingers used will be the thumb, index, middle, and ring finger.
You begin by plucking the open D string with your thumb. After this, use your ring finger to pluck the second fret (F#) of the high E string. Next is to strike the second fret of the G string (A) with your index finger, then finally resolving by plucking the third fret of the B string (D) with your middle finger. Once this is complete, simply repeat the figure again.
This pattern is one of the most basic and commonly used fingerpicking patterns, and appears in many songs. Don't worry if you can't play it very fast to start with, just focus on hitting each string with the correct finger.
Try New Chords
Many acoustic guitarists stick to open position chords or standard major and minor barre chords. There's nothing wrong with this at all, but it can get stale over time. To sound fresh, try mixing things up with some seventh chords. They're most commonly used in jazz, but can be used to add flavor in all genres of music.
They're called seventh chords because they incorporate the seventh interval from the root in addition to a major or minor triad. In other words, a major chord consists of the intervals 1-3-5. A seventh chord then makes that 1-3-5-7. An example of an A major seventh chord is as follows:
As you can see, it differs from the traditional A major open position chord in that it contains the first fret of the G string (G#) instead of the second fret (A). The reason for this is that G# is the seventh interval in the key of A major. (A, B, C#, D, E, F#, G#). By adding this interval, the chord takes on a jazzier tone.
There's also minor seventh chords, which take a minor triad and adding the minor seventh. An A minor triad would be A-C-E. By adding a minor seventh, it would become A-C-E-G. A minor seventh then, would look like this:
It differs from the traditional A minor in that it contains an open G string, because G is the minor seventh. This will also add a jazzier tone to the chord. These seventh chords exist in every key and all over the neck, so try them out to variate your playing!