Originally Posted by NoWhErE

Get yourself some egg crate foam panels (aka sound foam) to help cut out some of the ambiant sound.

Most audio software has noise reduction filters. Try adobe audition or Mixcraft if you're unsure what to use.

Also make sure the fan isn't blowing in the direction of the mic.

Blue Yetis are ok mics, but if you still can't cut out the surrounding noise, you'll need a better cardioid mic which won't pick up as much as the surrounding sound. Prices might e similar or slightly more expensive than the Blue Yeti, but it will do a much better job.