Borrow a crank puller from someone, pull the cranks, then unscrew the bottom bracket (officially you need tools for this, but I found a hammer and a screwdriver works too). Then take the bottom bracket apart and measure the length of the spindle (D). Also measure these sizes A, B and C:



Then go to Sheldon Brown's page, and see which symmetrical bottom bracket is equivalent to the one you just pulled out. Order one, together with the tool you need to screw it in (probably a Park BBT-22, or a Pedro bottom bracket tool--these are 20-tooth BB tools)

Boom! You're done.