Breakfast is eaten after getting out of bed in the morning and is usually small and light. When I lived with a host family, I would eat a bowl of cereal with milk, and drink some coffee.
Mid-morning, lots of people go to small breakfast bar for a mid-morning snack. At the language school, we'd go to a nearby cafe and usually our snacks would be a piece of toast with either butter, olive oil, or jam, and a cafe con leche.
Lunch is the biggest meal of the day and is usually eaten between 2pm and 5pm. At the host family's house, we'd have some soup or a salad, then something with meat, and then fruit for dessert.
Supper is usually at 10pm or 11pm and is much smaller than lunch. I only had half-board with the host family so I didn't get supper, but my friends and I would usually go for tapas in the evening at bars and cafes.
When I told some Spanish friends that in Canada, we usually eat supper at around 6pm, they were shocked and one of them said she can't imagine eating anything more than a snack at 6pm!
As for the basic food groups in Spain - olive oil, garlic, ham, and seafood. Pretty much everything "typically Spanish" has at least one of these in it!
Don't overtip in Spain! A service charge is often built into the bills in Spain and waiters do not expect the 15% to 20% that is customary in many parts of the world. A few coins and a "gracias" for good service at the end of a meal or drink is fine in most places.