Establish the time signature. The time signature appears at the beginning of a line of music and looks like one number stacked on top of another number. The top number signifies the number of beats per measure (a measure is the space between the vertical lines), and the bottom number signifies what type of note is considered "one beat."
For instance, 4 over 4 (also known as "common time") means that there are four beats per measure and a quarter note represents one beat. A time signature of 6 over 8 means that there are six beats per measure and an eighth note is one beat. The Music Facts Study Guide at scribd.com provides an easy visual explanation of time signatures.
Learn the types of notes. The different types of notes basically represent a fraction of a measure. A whole note is four beats long and is shaped like a tilted letter "O". A half note is two beats long and is an "O" shape with a straight stem. A quarter note is one beat long and looks like a solid dot with a straight stem. An eighth note is one-half beat long and looks like a quarter note with a tail. A sixteenth note is one-quarter beat long and looks like a quarter note with two tails. Look at "Basic Counting" by datadragon.com to see the note types and their values within a musical measure.
A dot next to any note adds half of that note's value, so a dotted quarter note is 1.5 beats long.
Learn the different types of rests. While a note signifies when to play, a rest represents when not to play. For each type of note, there is a corresponding rest with the same beat value. A whole note rest looks like a solid rectangle below the bar line (horizontal line); a half rest looks like a solid rectangle above the bar line; a quarter rest looks like a zig-zag lin; an eighth rest looks like a short slanted line with a dot attached and a sixteenth rest is an eighth rest with two dots attached. Refer to "Types of Rests" by datadragon.com to see these rests and their value within a musical measure. Adding a dot to a rest has the same affect as adding a dot to a note.
Count out-loud. It may seem silly to count music out-loud, but speaking and hearing the rhythm significantly improves your ability to process this new information. Again, refer to "Basic Counting" by datadragon.com for information on counting.
There is a widely accepted lingo for counting music. A measure containing four quarter notes would be counted "1-2-3-4." A measure containing eight eighth notes would be counted "1-and-2-and-3-and-4-and" and a measure containing 16 sixteenth notes is counted "1-ee-and-ah-2-ee-and-ah-3-ee-and-ah-4-ee-and-ah." Beginner piano theory books often have exercises specifically for counting aloud.
Write your own rhythms. After you are comfortable with the types of notes, rests and counting, create your own combinations of notes and rests and experiment with different time signatures.