I make this all the time, we use it for a dip, on souvlaki, and with meat. It is easy and tastes great. You can alter this recipe to make it to your taste.
You need a cheese cloth or some other cloth to strain the yogurt. I suspend it over a big bowl with a rubber band. Put about 4 cups of plain yogurt in the cloth and let the liquid drip out of it into the bowl. Spinkle about 1tbsp of salt on the yogurt. It takes about 2 hours.
Grate up a cucumber very fine, squeeze out as much of the liquid as you can, you should have about 1 cup of cucumber. Sprinkle some salt on the cucumber and set aside.
When the yogurt is thickened, mix in the cucumber, about 1 clove of crushed garlic (I used about a tsp of minced garlic from the grocery store, it depends on how garlicy you like it,) 2-3 tbsp of olive oil, and 1 tbsp of lemon juice.
As you make it you will be able to adjust the sauce to how you like it. We like it with a little more oil and less garlic, some people like it really garlicy.
It tastes great on warm pita bread, or with meat to make gyros, or as a topping for baked fish or meat.