Learning rhythm takes time. No pun intended. The general method that you use to learn rhythm is to tap a steady beat with your foot and sing different note durations against that beat. The basic method of counting out rhythms can be learned by anyone within a few hours.

With your foot, tap four beats, over and over as follows, making the beat on ONE the hardest.

Now sing the word "Da" and hold it for four taps of your foot. (Tap the top line, sing the "Da" line).

When you are able to do this comfortably, proceed to the next exercise, where you are going to create notes that are half the duration.

When you are able to do this comfortably, proceed to the next exercise, where you are going to mix notes of different durations.

When you are able to do this comfortably, proceed to the next exercise, where you are going to create notes that take up only one beat.

When you are able to do this comfortably, proceed to the next exercise, where you are going sing a different pattern.

Congratulations, you just played the start of jingle bells.

When you are reading sheet music notation, you group notes into individual beats so that you can play them. The beat is generally broken into smaller subdivisions, (in the above examples, further dividing the "Da" into more notes in a given beat), but since you have just learned to subdivide a 4 beat measure into individual beats, you can easily learn to divide a beat into smaller components too.

Now we will starting using note heads to represent the basic beat and the rhythm that you will sing. (Tap the top line and sing the bottom line, (use "Da" for the notes).

The following group of notes uses sixteenth notes to further subdivide the beat.

At this point you should be able to go to any sheet music and decipher the basic rhythm. When you see notes that are tied together and have the same pitch value, you don't play the second note, you simply continue playing the first note until the second note duration would be complete.

There are other subdivisions of beat beyond the 2 way and 4 way divisions that we have just completed. There are also divisions of 3, 5, 6 and so on. The 3 way division is known as a triple and the 5 way division is known as a quintuplet. Quintuplets are used constantly in Flamenco guitar.

See if you can tap a constant beat and divide it with "Da" notes 2 ways, 3 ways, 4 ways, and eventually a little faster, you can get 5 "Da" notes into one beat. Once you are able to subdivide the beat like this, you can generally decipher and then play any sheet music.

Dividing the beat while you are walking is another good way to learn. Try using your footsteps as the beat and think "Da Da DaDaDa Da" to divide the beat three ways. Try it with four divisions and then five. This is a great way to practice what you've just learned.