Spaghetti all'amatriciana is one of the most iconic Roman dishes. The traditional recipe calls for guanciale, a sort of bacon that comes from the fat around the pig's cheek. In a pinch you can replace it with smoked pancetta or bacon.
Guanciale is a bacon like product that is made from the cheek rather than the belly. You can substitute the guanciale with pancetta or smoked bacon if you can't find the real thing.
A few tips to perfectly cook pasta, as Italians would do: use a large amount of water, around 1L per 100g of pasta; add the pasta only when the water is boiling, stir, then bring back to the boil, stirring from time to time. Cook 1 minute less than the time suggested on the packet; drain and reserve some of the cooking water. Add the pasta to the sauce and finish cooking, adding some of the reserved water as needed. Serve straightaway! Also, never rinse pasta, you'll lose the starch needed to keep it moist.