Just a quick added note to the replies above. If this is your first single speed/fixie take note that the "proper tension" is just barely no tension at all. The chain should not be sloppy at all but on the other hand it should not go tight enough to feel "twangy" at any point over 5 or 6 pedal rotations. You need to check it over that many because as the rear gear and front ring move around and pass through all the possible combinations the "high" teeth, and there will be some, line up at different spots. So it may be fine for 4 or 5 pedal revolutions and then suddenly go G string tight on the 6th rotation. So check it properly. If it goes tight you'll feel it as a resistance in the crank arm and the chain will look like it is vibrating like a music instrument string. If that happens you have no real choice but to loosen the tension slightly or to try loosening the chainrings in the hope that it'll shift slightly and not create a similar tight spot elsewhere.