There's a common misconception that "you either have rhythm or you don't". I believed this when I was a young guitar player too. But I found that rhythm can be developed. Just like any other guitar skill. There are people out there that naturally have good rhythm. But I've played with plenty of skilled musicians who told me the same thing; they developed it.
Developing rhythm is just like developing good chord technique. It comes down to intentional practice. And with that, I'm going to show you a 5 step system that will help you develop outstanding rhythm (way better than average!).
5 Steps To Good Rhythm
The first step is my favorite. All you need to do is listen to music and tap along to the beat. Rhythm is all about feel. You have to feel the rhythm to play the rhythm. So spend some time listening to music and tap your foot along to the beat. You can also clap along, or bob your head. Do this for at least a verse and chorus for several songs.
You'll find it easier with songs that have a distinct BOOM-BOOM-BOOM-BOOM like electronic music. But it'll be harder with songs that are mellower or have more complex rhythm. This step seems passive, but if you're paying attention and really try to lock into the beat, you're training yourself to recognize good rhythm and timing.
The second step will involve a metronome. We're going to clap to the beat of a metronome. Start nice and slow. Anything you can do slow you can do fast. So set your metronome to 58bpm (beats per minute) and clap along. You'll be clapping as perfectly in time with the beep as possible. These are the down beats.
The next part of this exercise is to add the up beat. It's the "and" in 1-and-2-and-3-and-four-and. This should be a little more challenging. Especially at a slow tempo. You can think about it as clapping twice as fast as you just were. But try to make the downbeats just a little louder.
This is a great exercise to do before you even pick up your guitar, and I recommend that you do it daily until it feels natural.
The third step involves your guitar. You're going to do a similar exercise to the second step. Mute your guitar strings with your left hand. You're going to strum along to the metronome, but with muted strings. Strum down to match the metronome. 1-2-3-4, 1-2-3-4. From there you can add the up strums on the up beats. 1-and-2-and-3-and-four-and.
Parts 1 and 2 of this exercise had you strum once and twice per beat. The last part of this exercise is to strum 4 times per beat. You can count this by building on the counting system we've been using.
The 'e' and 'a' are what you'll add. I would recommend jumping to 4:20 in the video to hear this if you're unfamiliar with this counting system.
The fourth step will get you to move your left hand. Just like you would if you were strumming chords. You're going to keep your strings muted, but you'll get used to moving your left hand while strumming. Here's the timing of the pattern:
Now, on the "1-e-and-a-2-e-and-a-" keep your left hand around the 5th fret. Then on the "3-and-4-and" jump down to the first fret. Go back to the 5th fret for the "1-e-and-a-2-e-and-a-", and then jump up to the 7th fret for the "3-and-4-and".
(Check out 4:55 in the video to hear this.)
Make sure to practice this with the metronome and really lock in the timing.
The fifth step will bring in chords. Once you're feeling good about the pattern in step four you can add in chords. You only need 2 chords, and the chords don't really matter as long as they're chords you're familiar with. I'm going to use D and A7.
On the "1-e-and-a-2-e-and-a-" I'm going to play a D chord. Then on the "3-and-4-and" I'll change to an A7.
Same rhythm, now with the added complexity of chord changes. You can speed this up along the way, but make sure your timing is really solid before changing the tempo.
There will come a time where you will be strumming a rhythm and it'll be right on that you're not even having to think about it ...
Tips For Developing your Rhythm
As you're going through the exercises make sure you're getting one step down before moving onto the next. You might be tempted to skip straight to 5 and try it out. But the incremental steps will pay off big time in the long (and short!) run.
Most of these exercises are played to a metronome, but I don't want you to be so focused on it that it's holding you back. If you're finding it too challenging to play along with the metronome, then work on the exercises without it. Once you're feeling more confident come back at it with the metronome and find a good starting tempo.
Also, try to vocalize the counting as you're playing. It's helpful to count the beats as you're playing. Use your body too. Tap your foot, sway, whatever you need to do to feel the rhythm. You'll eventually get to the point when you're feeling the rhythm, and not counting it. And that's one of the best feelings!