Know the rager, tinseth or Garetz ( spelled this one wrong ) method if calls for ibu's. Make life easy buy beersmith or similar enter your hops into the program and done. Life is good.
You can use online calculators, software, or do it manually. I use BeerSmith to calculate the IBU's but you can do it manually by going to John Palmer's website and reading up on the math.
Just a side note...the % of alpha and beta acid in hops can change from year to year, growers in different areas, etc. Check the % on the bag and use that in your calculations. It usually doesn't change much, but I've had as much as a 4% change on some varieties (Columbus for example).