You have framed the question correctly, which means you are already 80% of the way to knowing! How do we oppose capitalism? Well, together! Individuals can do very little against the dominant system, but in an organized group we can use tried and true means - of organized withholding of labor, of takinh direct action, of increasing the size of our ranks, of educating ourselves and each other, and, usually out of necessity, arming ourselves.
So the question then becomes: okay, duh, we need lots of people working together, but how do we do that, what are better ways of doing it than others, and how do I get involved? This is a very important question because historically there are examples of success, failure, and outright counter-productive movements that all had this same stated goal. This is every dedicated anticapitalist's biggest thing to fret about: which lessons from history apply to us and which do not? What is best in your locale may not be what is best in someone else's and there may be many pathways that are better or worse than the other. How do you choose which to avoid and which to embrace? Where do you, personally, fit into the equation?
The othet answers have the right gist: personal education and joining and contributing to an organization.
There is a substantial catalog of political theory, history, philosophy, media criticism, and practical organizing skills that are almosy entirely untaught in capitalist-dominated school systems. Reading a good chunk of that catalog is important for choosing the right actions personally as well as contributing to the decisions made by an organization. You don't have to read all of it before you begin work in an organization, but you should start reading ASAP. I recommend reading Blackshirts and Reds first, it is very short and digestible and provides a good framing by which to question the modern history of capitalists, socialists, and fascists. From there I would gravitate to Marx and media criticism, such as Heinrich's explainer (which I would eventually compare to works criticap of Heinrich) and FAIR.org or the podcast Citations Needed. Add some Engels as well. From there you can branch out in any direction you would like, but an understanding of the October Revolution, its precedents, the USSR, and a critical approach to its critics is helpful for understanding what the hell everyone is talking about in a given organization (and people are often saying incorrect things on these topics). There is a recent Liberation School series on the topic, effectively by PSL, that I recommend. Others to look into after this: Goldman, Gramsci, Mao, Che, Kalecki, Amin, Fanon, Freire, Bobby Seale, George Jackson, Michael Hudson. This will provide some of the "Greatest Hits", albeit Western Centric. Dedicating time to the history of every socialist revolution is valuable. It will take yeara to read all of this and this is normal.
The other step is to join and organization. This gives you the opportunity to learn practical skills for getting people involved, educating people, being educated by others, and taking action. Not all anticapitalist organizations are created equal and there is a tendency for infighting between them. Some are actually highly counterproductive, so this isn't just pointless infighting though a lot of it is pointless. So long as you avoid abusive organizations that burn you out (or worse), being in any org is better than trying to pick the best one to join on your first try. I will suggest avoiding these kinds of organizations: Trotskyist, non-profits, Maoists that are up in each others' business, liberal identity politics groups (socialist/Marxist identity-focused groups can be good though!), and any group that spends most of its time on things like electoral politics and letter writing campaigns, i.e. what capitalists want their opposition to waste their time on. The first 3 groups are the most likely to be abusive and burn you out. The last is basically not actually anticapitalist at all even if they claim to be. To find options for local organization I recommend using a combination of attending events that sound cool and wors of mouth recommendations. "Anti-imperialist" is a decent indicator that a group is fairly cool, though it is not a guarantee. Go to a few events and feel them out. Protests, teach-ins, hosted political movie watching events, rallies, etc. You want to focus on the groups organizing these things, not just attending them, and only rule them out if they meet the above ezclusionary criteria. If they can't be ruled out, ask them how you can get involved. Any good organization will be very excited to loop you in and get you attending meetinfs and reading sessions within a few weeks.