Begin with 4/4 time. This is the easiest and most straightforward time to learn how to read note lengths in sheet music. 4/4 time means that there are 4 beats per measure and each beat is equal to one quarter note. The top number determines the number of beats and the bottom number determines the note that is equal to one beat. So there are 4 beats and 1/4, or a quarter note, is equal to one beat.
Begin with the longest note. This is typically the whole note. The whole note looks like an "O" on the staff. A whole note takes up all the beats in the measure. So if you're working in 4/4, a whole note lasts for 4 beats.
Add a line to the whole note and you get a half note. A half note looks like a lower case "d" on the staff. A half note is just like the name implies, half of the beats in the staff. So for 4/4 time it would take 2 beats.
Fill in the circle and you get a quarter note. A quarter note looks a lot like a half except the circle part of the "d" is colored in. A quarter note is equal to one beat in 4/4 time.
Add a flag-like tail to the top of the quarter note and you get an eighth note. An eighth note is worth half of a quarter note. In 4/4 time, an eighth note is worth half a beat. Attach two flags to a quarter note and you get a 16th note. A 16th note is worth a quarter of the beat. You need 4 sixteenth notes to make up one beat in 4/4 time.
Look at different time signatures. 4/4 is the most common time signature but it is not the only one. There are dozens that the sheet music can be in. There is 3/4, 6/8, 5/8, 2/2 and many others. To figure out how the notes are affected by the time begin by looking at each part of the time signature. The top number tells you how many beats there are per measure. For 3/4 have 3 beats per measure, 6/8 have 6 beats per measure and 2/2 has 2 beats per measure. The bottom portion tells you what note is equal to one beat. 3/4 means that a quarter, or 1/4th, note takes up one beat, 6/8 time means that one eighth, 1/8th note, takes up one beat and 2/2 means that one half, or 1/2, note takes up a single beat.
Apply the time signature to the rest of the notes. For 3/4 time, a whole note is 3 beats, a half is 1 & 1/2 beats, a quarter is 1 beat, an eighth note is 1/2 a beat and so on. For 6/8 time it gets trickier. A whole note is equal to 6 beats, a half note is equal to 3 beats, a quarter note is 2 beats, an eighth note is 1 beat and a 16th note is 1/2 a beat.