There are several parts of a song. Your song can include all of them or none of them. It really all depends on you. There are standard layouts of these parts that are used in most songs, however, so in order to understand how most songs sound, you'll need to understand the parts. They include:
An Introduction - this is the section at the beginning which leads into the song. Sometimes it might sound different from the rest of the song, might be faster or slower, or it might not exist at all. Many songs do not have an introduction, so don't feel like you have to use it. A Verse - This is the main part of the song. It is usually fifty percent to twice the number of lines as the chorus but it does not have to be. What gives away a section of a song as a verse is that the melody is the same but the lyrics are different between the different verses. A Chorus - The chorus is the part of the song that repeats without changing: both the lyrics and melody are unchanged or nearly unchanged. This is usually where you try to fit the catchiest part of your song (usually called the hook). A Bridge - The bridge is a part that exists in some songs but not all. Usually coming sometime after the second chorus, the bridge is a part of the song that sounds completely different than the rest of the song. It is usually short, just a line or two of lyrics, and will sometimes lead into a key change.