Gatsby met Daisy during his enlistment.  He was stationed at Camp Taylor and Daisy was very popular with the young soldiers.  They dated a while but he was sent off to war and Daisy didn't wait for a poor soldier, she married a rich Tom.  After Gatsby got out of the Army he vowed to earn a lot of money so that he could win Daisy.  He realized that he would never have a chance to win her love and her commitment without money.  He chased the American Dream by becoming a crook.  He went to work with Meyer Wolfsheim who was thought to be guilty of fixing the results of the World Series in 1919.  Basically he is a drug dealer.  Not the kind we see on the streets but a major player and he makes a great deal of money.  Yet, it is not enough to earn Daisy's commitment or pull her away from Tom.

These are two separate questions, so I'll only answer the first. Gatsby knew Daisy from Louisville-he was stationed there for a brief period. Daisy was quite popular with the soldiers at Camp Taylor, which is how she met Gatsby. For him, Daisy represented the American Dream, everything for which he was striving. After a brief love affair, he planned to marry her, but his orders came to ship out. She was prevented from meeting him to say goodbye, & when Tom shows up with a $350,000 pearl necklace, her marriage plans suddenly change.