Begin with the most common form in popular music, the "ternary song form." We think of this as A A B A. (Let's use Cole Porter's "Anything Goes" as an example.)
Look at the initial musical statement, A. This is usually eight measures (bars) long. (This ends after the first "Anything goes.")
Examine the second A. It's an exact, eight-measure repeat of the music in the first A, but the lyric continues to move along. (This brings us to 16 measures, to the end of the second "Anything goes.")
Note that this second A is critical in two ways: the listener's ear perceives the melodic material better after hearing it twice, and the lyricist retains the rhythm of the first A so that the musical idea is not disturbed.
Go on to B, the "bridge," an 8-measure departure from the musical material of the A section and its repeat. (We've now used up 24 bars, through "silly gigolos.") Listen to how the composer or lyricist departs from the rhythmic and melodic material used in the A sections to draw the listener's ear away from familiar ground and establish a fresh idea.
Finish with A in the final eight-measure section. This is a return home to the familiarity of the beginning melody and rhythm - leaving the wilderness of the bridge. (This brings us past the third and final "Anything goes.")
Note how this A A B A form seems deceptively simple. In fact, it can be quite intriguing in the hands of a master composer-lyricist like Cole Porter.
Study another common, basic form - A B A C. Analyze "Gigi," with lyrics by Alan Jay Lerner and music by Frederick Loewe. "Gigi" is written in this form.
Make an intensive study of other lyricists and other styles. Although creating a song is a very personal endeavor, you will increase the colors on your own palette and foster the growth of your own versatility by doing so.