Once your wort is cool, put some in your hydrometer cylinder, or the tube it came in if you don't have one. Make sure the sample is close to 60 deg F. You can put it in the fridge for a while. Then gently put the hydrometer in and give it a little spin to knock off any air bubbles clinging to it and take a reading at the meniscus (the lowest point in the curve that the liquid forms at the surface). Use the specific gravity scale. If the sample isn't 60deg, your hydrometer should have came with a chart for temperature correction. (should be anywhere from 1.040 to 1.060 depending on the recipe.