Choosing a political party can be very difficult, particularly for someone who is willing to spend the necessary time reading about and learning more about the issues than is typically available to the average TV-watching news consumer. It can also depend a great deal on where you are joining a party, given that political parties serve very different functions in different countries and different political systems.
Looking at the two-party system in the US may be instructive even if it will not address all the details of joining a party in a parliamentary system where there are far more parties to choose from catering to more specific and nuanced interests.
It is important to begin with a consideration of what you find particularly important in a party. Are you focused on one or two specific issues, issues that are important enough to you to accept compromised positions on other things? Do you feel that being pro-life or pro-choice is so important that it absolutely must be a part of your party's platform? Being a one (or two) issue voter can make the choice more straight-forward given the general divide, but it can also lead to conflicting feelings when considering how that affects other issues.
It is also important to view the history of each party to understand not only what you are going to be getting into but also what each party has undertaken and accomplished as a group. It can be very difficult to distinguish one party from the other once you look more closely at how positions on certain things and how parties themselves have evolved over time.
So a careful, deliberate and well-informed choice will take some reading and some investigation. It can also be helpful to speak with people you respect about their choices and the process they went through in order to arrive at the choice they made.