Tune your bass to standard EADG tuning. Use a tuner if necessary. "Iron Man" is played at on a 4/4 beat, so be sure to tap your foot to the pulse or use a metronome set at 4/4 to help you keep time. "Iron Man" consists of 14 notes in the main riff.
Listen to the recording of the song to hear how the bass line is entwined and doubles the main guitar riff. Play one note on the open E string, then move the G on the E string's third fret for two notes. The next two notes are the A on the fifth fret.
Play the next six notes of the riff the 8th and 7th frets (C and B). These notes are played faster in succession than the previous notes; use slides or hammer-on/pull-offs to make the notes sound smoother if needed. For the next two notes, play the G on the third fret again and the final two notes on the A (fifth fret).
Listen to the recording and determine the chorus and breakdown sections, which are variations of the riff and should be easy to determine after you've learned the main riff. Add harmonic interest to the line by using slides between the notes or use double stops (chords) to aid the guitar during the chorus sections.