Depending on what your startup / website is offering, you should focus on different channels for customer acquisition.
As Dan suggests, you can usually categorize the different customer acquisition channels into paid vs. non-paid (meaning you're not directly paying to acquire traffic, not that the traffic comes without you having to spend time and money on getting it). Customer acquisition is the biggest problem startups face by far. Many channels are awfully noisy. Which channel makes sense for you depends entirely on what your startup does and where your audience resides and spends their time.