With regards to pigs foods, the basics are
They can eat ANY vegetable except potato
They can eat most non acidic fruits
Don't put vitamin drops in water as it will change flavour
Fresh straw for food is needed every day so that they get their vitamin c which is essential
They love acidic fruit like oranges, but give half once a week otherwise the acid will cause blisters
Guinea pigs go mad for parsley so this is a fab treat every once in a while
Dont give them iceberg lettuce as this can give them tummy ache
In summer months they love dandelions and dandelion leaves and will eat these in abundance.
The best food you can give them is 'Guinea Excel' it comes in a bright orange bag.
Other pointers are
In the summer when it is hot a house brick in the cage is great for them to lay on (try and rub corners down if you can)
Check their claws every week, some grow more than other
Let them have somewhere to hide, whether it be an old box or a proper house
In summer months if you can let them run about on the grass, they will love the fresh air and amount of grass they can eat (make sure its covered so cats, dogs, birds cant get them (I lie at the side for an hour and read the book)
Main things
If a guinea pig lives outside, the estimated age is 3-5 years
Inside it is 4-9 years as they are protected by the elements
In summer if I'm nipping out and I know it wont rain i put them outside in their cages (i have 3)
Any questions feel free to ask