The short answer
Use search when demand is explicit, Reddit and communities when the problem is discussed, LinkedIn when buyers have professional identity, and paid ads when you can measure a qualified lead or signup event.
Answer
How do I choose startup marketing channels?
Choose startup marketing channels by starting where buyer intent is already visible, then testing whether your message can earn attention there at a cost and cadence you can sustain.
How Wally helps
Wally researches your market and product, suggests channel opportunities, drafts channel-specific campaigns, and helps you compare what to do next instead of guessing from a blank page.