1. Get in where you fit in. What are your passions, strengths and weaknesses? We should always be challenging ourselves, but we should ALSO be figuring out what we're good at and where/how we can do the most good with our talents. There are a lot of different struggles out there, and any one individual can't win them all; to some extent, we have to choose a path. Important questions to ask: what issue do you care most deeply about? Why? How does it affect you? How do you see yourself fitting into that struggle?
2. Dive in. Find an organization. No matter how brilliant you are, you can't do this alone. There is power in organization. Find a good one. Use the database here, or just ask around. Explore. The odds are good that there's a group out there working on the issues that you are passionate about. They may already have plans and/or funding. Join up and get involved!
3. If you have to, create an organization. Don't reinvent the wheel if you don't have to, but if no one is working on the issue(s) that you care about, start your own group. It's just a matter of setting a meeting time/place and then spreading the word (email, facebook, flyers, posters, etc.). Once you have a small core of people, you can start to figure out some kind of plan of action. You can grow.
4. Balance personal growth with working for institutional change. Some activists are all about self-work-- reading books, watching documentaries, learning and going to workshops and all that. Other activists are all about the struggle-- rallies, marches, lobbying, whatever. We need both. Critical self-reflection that never leads to action is pointless. Radical action without any critical self-reflection can lead to some bad things.
5. Get power. This is something a lot of progressives aren't comfortable with, but it's important. If you really want to change things, put yourself or people whom you trust in positions of power. Maybe this means running for office. Maybe this means choosing a social justice-oriented career path. Maybe this means wearing a suit sometimes. Maybe not. Whatever it means, it's important to have long-term goals on top of short and medium ones.
So what else? If you're an experienced activist, how did YOU first get involved? What advice would you have for beginners?