Learn to read guitar tablature. Tablature is a simplified musical notation for the guitar that's commonly used for playing and writing down pop songs. There's a tablature, or "tab," available free online for almost every Fall Out Boy song. A tab has six horizontal lines, one for each string of the guitar. The top line represents the low E string, and the bottom line represents the high E string. Numbers are written on the lines corresponding to what frets should be played on each string. Often, chord names are also written above the lines, and the lyrics are written down to help you keep your place in the song. Tablature gives you the order to play the notes in but not the timing of each note, so you'll have to listen to the song. See the Resources section for a detailed guide to reading tabs.
Find some tabs for the Fall Out Boy song you want to play. Tabs are written down by amateurs who want to share their knowledge of how to play a song with others. Often many different versions exist for a single song, and some are more accurate than others. Lots of websites collect and list guitar tabs. There are links to a few in the Resources section.
Read through the tab while listening to the song to get a feel for the structure and rhythm, then try playing it. At first, listening to the song while playing and reading the tab will make your life a lot easier, and help you notice parts of the tab that don't seem correct. If something in the tab sounds wrong to you, don't be afraid to experiment until it sounds right, or look for a different tab.