what action each character could have taken to make sure their dream came true ?

I think the sad reality is - nothing. Although each of the characters has a dream and direction they would like to pursue, none of them have the conditions to be able to do so. George would like to settle in a place of his own, but scrubbing for work during the Great Depression is a hard, if not impossible way to 'roll up a stake' and get enough money to buy a place of his own. Plus, he has Lennie to take care of. Candy may have money to buy a place, but his disability means that he cannot succeed independently. Curley's wife fell for the dance hall line of being pretty enough to be in movies at a time when many young women were seduced this way. Crooks as a black man would not live to see anything approaching equality or freedom for his people. Lennie wouldbe as destructive and dangerous on his own property as he is on the ranches he has worked.

The reality for each of them is that there is a flaw in the fabric of their dream which makes it impossible.