Fixing a marriage is no easy task but you can start to make improvements by making a concerted effort to avoid conflict, especially when it comes to the small stuff.
Fixing a marriage takes both people working on it. Both have to be willing to put the other first. Both have to be willing to compromise. Affection helps too.
It depends on what the problem is. The first thing in any situation is to be honest with your spouse about what you feel is wrong with your marriage. You have to have two willing parties to fix anything. Seek counseling to sort it out if you are in together.