There are two main factors that determine how good the surf will be. First, there are the conditions that lead to initial wave formation. These determine the quality and size of the ground swell that will hit the surf spot. The second main factor is the local conditions. These have to be right to ensure the swell can create quality waves.
Wind direction has a huge effect on a wave. The ideal wind for a surf spot is a light offshore wind. An offshore wind blows from the shore, smoothing out the face of the wave and helps hold the lip up — the things a surfer is after. An onshore wind will make the wave break irregularly and close out, making the wave unsurfable.
Comparing onshore and offshore waves
Swell Exposure - The break needs to have exposure to the swell. If waves no come, Billy no surf. One spot may be flat while one just round the corner could be classic with its beach facing a different direction.
Sea Floor - The sea floor has a huge effect on the size and type of waves that will form. A gently sloping beach will cause the waves to slow and possibly break too early to form a quality surfable wave. A steep floor causes waves to overshoot their normal breaking depth. The contours of the sea bed are also important. If waves hit a flat, featureless sea-bed, then closeouts are likely. Find out more about this on the "How Waves Break" page.
Tides can make or break a surf spot. At some places the tidal range is so large that your first surfing purchase should be a tide table and not a surfboard. It may be that a surf spot is only surfable for a few hours a day.
Waves will break at a certain depth depending on the wave height. If the depth of water increases or decreases as the tide changes, waves will no longer break at the same point. Shallow reefs are particularly susceptible to changes in depth.
Some areas experience big tidal ranges (e.g. Europe), and being aware of the required tide at a certain spot is essential to be there at the right time. Tide tables are essential.