What do I do if my DNA test reveals my dad isn't my biological father?
First: I'm sorry. This is one of the heaviest discoveries that comes out of consumer DNA testing, and you are not alone - there is a name for what happens when people find out this way (NPE, "Not Parent Expected"), and there are large support communities online specifically for people in your situation.
Practical, in this order:
One - confirm. Sometimes the algorithm gets it wrong, especially for distant relative matches. If you haven't already, contact the company and ask them to re-check the analysis. Upload your raw data to a second company and see if the result holds.
Two - don't act fast. Don't confront your parents the day you get the result. Don't post about it. Don't reach out to potential biological relatives yet. Take a week, minimum.
Three - find your people. The NPE Friends Fellowship, DNA NPE Friends on Facebook, and similar communities are full of people who have lived through exactly what you are going through. They will help you think about timing, what to say, what to expect.
Four - therapy is genuinely useful here, ideally with someone who has worked with genealogy-related family discoveries. This is a long process, not a single conversation.
Five - your dad is still your dad, in every way that matters except the biological one. That doesn't make the biological fact less true, but it also doesn't unmake the relationship. Whatever you decide to do, those things can coexist.
Take care.
Echoing Sunita's advice. The single most important thing in the first week: do nothing irreversible.
I have worked with families on this. The discoveries are real, the feelings are real, but the people involved - your parents, your siblings, your possible biological father, his possible other family - are real too. A few quiet weeks of processing before any conversation tends to lead to better outcomes for everyone, including you.
Many genetic counsellors will see you for a standalone consultation on this exact issue. You don't need a referral. Find one in your city.