The Rebel Alliance needs weapons, you need credits, seems like a fair deal and if anything does go wrong, well, there are other benefits to the cargo you're hauling being made up of blasters.
After the Battle of Yavin, the Alliance needs currency to keep the war effort going. You and your tramp freighter crew are recruited for the treasure hunt.
Your boss, a Twi'Lek called Zakesh, has one last job for your crew, and then you will all be the proud owners of your own ship. He says it should be a simple job, but you know how it is.