How to cook a fair Pasta, the Italian way
Depending on your recipe, you might have to start preparing sauces or other accessories before making the actual pasta.
The way of boiling pasta applies to most every pasta in general.
When the water is still cold, add some salt to it. The quantity of salt depends on the amount of water and the salt's particular intensity (some salts are "lighter" than other ones). On average, for every 200 grams of pasta you need 4 liters of water and 2 tablespoons of salt.
Moisten the salt with water until it is no longer visible. When the water finally boils, throw in your pasta and start measuring the time. Do not put pasta in the water until it has started boiling, or you will get something very distant from what is generally considered satisfactory pasta.
The length of time differs for each kind of pasta (generally, the box suggests a boiling time), and goes from 5 minutes for thin spaghettini to 12 and up minutes for some thicker kinds. A bit less time than suggested will produce a firmer pasta (al dente - heavy to digest, however) while a bit more time causes pasta to be more flexible. It is a good idea to follow the suggestions on the box until you are sufficiently experienced in making pasta.
Fresh, egg-based pasta (pasta all'uovo) takes very little time to cook. Fresh noodles hardly need a minute after the water has returned to the boil; filled pasta like tortellini need only a few minutes to cook through.
Many experienced cooks like to sample a piece of the pasta while boiling it (spizzicatura - they "fish" it out with a slotted spoon) to stop cooking just at their desired consistency.
When boiling time is completed, separate pasta from water with a strainer, put in a wide dish and add everything else. A suggestion: a simple combination of butter and Parmigiano Reggiano, or some oil, parsley or basil and fresh garlic, will be as tasty as more sophisticated preparations. Eat the pasta hot, in the next 3-4 minutes, unless the recipe says differently.
see also the Wikipedia Cookbook, cheese and other HowTos