First, you have to determine what political issue or issues are most important to you. Some of the "hot button" issues today are health care, immigration, Social Security, gun control, student loans, and taxes.
Say you decide health care is the most important issue to you. What would you like the national policy towards health care to be? Would you like, for example, a single payer system? If so, look to see if either party supports it. If one party modified its platform to support it, would that make the decisive difference for you to vote for candidates from that party? Or would you perhaps like no government support for health care? Does either party support that? It one did, would that make the difference in giving it your vote?
Or perhaps you don't want to have to worry about supporting Mom and Dad when they are too old to work. In that case, you might want to get behind a party with a platform plan of securing Social Security. Would that modification get your vote? Or maybe you want to abolish Social Security? Would that modification get your vote?
Or are you basically OK with one major party, but would like a few tweaks to their platform? For example, are you basically happy with the Republicans or the Democrats except for one or two things that make you very hesitant to vote for them. Think about what those one or two things are: would the Democrats appeal to you very strongly, if, say, they opposed abortion? Would the Republicans appeal to you if they just changed their stance on gun control?
Just think about how you would like the country to be different and what areas the parties could address that would bring the changes that would earn your vote.