I understand your fears of fascism, but fascism cannot be stopped at the ballot box. Fascism is the result of the capitalist class removing any and all civic protections. It's an extra-legal action, one that does not happen at the ballot box but instead through the capitalist class itself nakedly taking away civil liberties. Voting for Democrats or Republicans does not speed up nor slow down fascism.

As for where to start, both Freedom Road Socialist Organization and the Party for Socialism and Liberation have two tiers of membership, general membership which is low commitment in time and money, and cadre membership, which requires significant time and money. See which is more active near you (almost certainly PSL, but varies), and consider joining the general membership!