Well, for 5 persons you need at least 600 gr. of fresh clams and 600 gr. of fresh mussels. With "fresh" I mean that they have to be alive. Put it in a pretty big pan (you will put pasta inside it later, so it has to be big). Turn of the fire and after few minutes clams and mussels will expel their water. Let them cook for some minutes, until they will open. If, after 10 minutes, some clams or mussels are not open it means that they were dead before you cooked them, so throw them away. It's important that molluscs are alive before you cook it (Vegans, don't watch please and keep eat carrots
). Remove molluscs from the pan then, and keep their water in a container (you will add it later to pasta). In the same pan (now empty) put olive oil, 2 slices of garlic and 1 chili pepper and let all fry for just some minutes, until garlic gets a "golden" color. Remove garlic and add coocked clams and mussels (but don't turn on the fire). In another pot, cook spaghetti (about 500 gr. for 5 persons) in salt water. When pasta is coocked, drain it and put it in the pan with clams and mussels. Turn on the fire and add molluscs water I mentioned in the fifth line. If you like it, add also a bit of pasta cooking water. Mix with 2 forks for some minutes. Add some chopped fresh parsley and you can fill plates. Obviously you need also some iced white wine
Buon appetito