Know what to feed your Guinea Pig- and what not. In this paragraph I'll show you what your Guinea Pig may and may not like, along with some basic necessities!
Water- An Obvious necessity. If you feed your Guinea Pig lots of fruits and veggies, then your Piggy will drink less, but if you only feed your Piggy pellets and hay, then water is a definite!
Grass- First, make sure you know your lawn does not have pesticides. Put your Piggies out in their playpen during the Spring or Summer months and let them play! Place them in the shade, so they don't get over-heated. Make sure you have a dark place for them to sit in to keep them cool! Guinea Pigs love to eat grass so if you move the playpen around daily, they'll keep you lawn nice and short- fertilize it, too! 
Hay- Piggies love to eat lots of hay! They play in it, sleep in it, and eat it! Your Guinea Pig won't like straw, so don't try it. Piggies prefer hay over straw any day. Guinea Pigs under 6 months can be given Alfalfa hay, but for older Piggies, it contains too much calcium and can harm them. Give them Timothy Hay. Timothy Hay is very good. Give them Alfalfa Hay once a month for a treat, for protein. They must have hay 24/7.
Vitamin C- Guinea Pigs cannot produce Vitamin C, like humans, as you read in the first paragraph. So Guinea Pigs need to obtain it from their food. Because if they don't, Scurvy is a possibility. Scurvy is a disease caused by low-levels of Vitamin C. So feed your Guinea Pig lots of oranges and other food products with Vitamin C!
Fruits and Veggies- Some fruits are high in Vitamin C, and others are just treats. Give them oranges daily, as they seem to have a liking to them, and are very high in Vitamin C. Everything you give them should be about room temp., including their water. Try giving them a variety of fruits, like strawberries, oranges, seedless melon, apple, banana, peaches, etc. ( Don't Feed citrus in quantities, because they can lead to mouth sores.)
Pellets- Pellets are needed almost all times of the day because they provide lots of IF(I have no clue what that means). NEVER use pellets mixed with seeds, nuts, dried fruits, vegetables, etc. These are usually high in fat, and/ or the Guinea Pig could choke on them.