This question addresses how to keep pasta from sticking after it has been cooked. This question is what I want, but was wrongly closed. All the answers from the first address the sticking post-boiling, and my problem is the sticking during boiling. How can I prevent this from happening?
Simply put, you have to stir them the first three/four minutes of cooking. Be sure to boil them in enough water (at least 1 liter per 100 grams). In addition to that be sure to buy some good quality pasta. From my knowledge pasta tends to stick when the wheat used is of poor quality. A good pasta should not be transparent when seen in direct sunlight, but it should be of a nice pale yellow color and opaque. Good Italian pasta brands that you can find outside of Italy are De Cecco (which I fear might be quite expensive, and probably not so easy to find), or Barilla (easier to find). Both of them has cooking instructions printed on the package. You might want to cook them for 2-3 minutes less than what's printed, then eventually cook them the remaining couple of minutes together with your sauce.
I find it's most important to make sure that it's moving well at the beginning; a quick stir after adding the pasta does wonders, and sometimes a second stir after the water comes back to a boil.
I also try to make sure the pasta goes in with a little bit of separation. Strand type pasta are the most difficult, but if you hold it in both hands, and give one a twist, it'll cause it to fan out, so you can get it to disperse better when it first hits the water. For most others (shells, penne, orzo, etc.), I pour it in over a few seconds rather than just completely upending the box.
Sufficient water for boiling so there's space for the pasta to move also helps.
You can also try adding olive oil to the pot of water before it boils. the olive oil and occasional stir will help the noodles from sticking.
This answer to your referenced question should address your issues.
The important part is step 3:
Stir pasta for a couple of minutes after pouring it in the hot water
Stirring frequently in the first few minutes, as well as using plenty of water will address your sticking issues.