First of all, make sure you have the water-packed tofu, not the silken tofu that comes in the aseptic boxes.
Heat a non-stick pan or a very well-seasoned cast-iron pan over medium heat, if you have an electric stove, or low to medium-low on a gas stove. Cut your tofu in half lengthwise. Place it between paper towels or clean dish towels and press gently (hard enough to get some of the moisture out but not hard enough to squish it). Cut into about 16 triangles or strips.
Add the tofu pieces to the pan. Leave plenty of room around each piece (you might want to cook it in two batches). Cook VERY SLOWLY over LOW TO MEDIUM heat. Slow cooking is the key to ensuring the tofu doesn't stick to the pan and that the water has time to evaporate so that the tofu can brown. Do NOT use oil. What you want to do is leach all the water out of your tofu in a dry pan. As the tofu cooks, frequently use a spatula to press down on the top of each piece. You should see some water squeezing out and sizzling in the pan.
When the bottom sides are firm and golden, carefully flip the pieces and repeat the same process on the other sides. They are done when they are firm and golden on both sides.
Now that your tofu has given up most of its water, it is perfectly ready to soak up any marinade like a sponge. Place the cooked tofu pieces in your marinade of choice, stir, and let sit for about half an hour. Now your tofu is ready to eat or to use in a stir-fry!
Uncooked leftover tofu should be placed in a container that has a lid, submerged in water, sealed, and stored in the fridge. Try to remember to change the water every day or two until you use it. Cooked tofu can be stored like any other leftovers.