Guinea Pigs are far shyer than Rabbits but they can be handled easily as they do not tend to bite. But they are likely to scurry away under something for periods of time rather than spend time with their owner. Having said that if handled well and with confidence they will make a rewarding pet. You could possibly team them up with a friendly Rabbit for company but beware of bullying and make sure the cage is large enough for them?
Food and Water
Guinea pigs have an unusual metabolic quirk, they are unable to produce Vitamin C from their food. So it must e present in their diets. Buy a reputable ready made food for them and you will then guarantee a balanced diet for them. They can also eat a variety of green stuff and vegetables, Broccoli and other brassicas and spinach contain good levels of Vitamin C. These come in handy in the winter months when their favourites Dandelion leaves aren't so readily available. Root vegetables are also good, carrots are always welcome. You must feed them daily and change their water, the drip water bottle being best.
Accommodation
Guinea Pigs can live inside or out, but it will need a sturdy outdoor hutch that is big enough to move around but they do not need to be as tall as a Rabbit hutch
You can use sawdust for the floor. Straw, hay and shredded paper they are all acceptable for bedding. When in their hutch you usually find that they use one particular area to use for the toilet, make sure this is clear of foodstuffs and has more paper underneath than normal. Placing a rubber product or vinyl flooring on the floor of the hutch will save the wood from eventually rotting.
Make sure that if your Guinea Pig lives inside it can't chew any electrical wires or furniture. Their housing should be secure and enclosed to stop sawdust from spreading. If your Guinea Pig is outside make sure their hutch is also kept away from dogs, cats, foxes and other animals. The hutch must be waterproof and kept off the ground so that it doesn't get damp, cold or draughty. A good hutch should have two compartments, one for sleeping so that they feel secure and another that's lighter and more airy where they can see out but which is protected by strong wire netting. Also make sure they have something for them to gnaw, to help their teeth.
In the winter always try and protect your hutch from the elements, if possibly placing the hutch in a garage or out house is a good 
idea. And make sure the water does not get frozen so don't fill it to the top!
Exercise
Guinea Pigs need daily exercise or they become bored. Remember to check that your garden is safe for them to play in as they can squeeze through small places so it is best to have a special run either brought or made for them to exercise in. If they will be out in a run for long periods always ensure that there is a shaded area for them to rest and to protect them from the sun and rain. Make sure that there is always water available and that the run is secure form any predators such as dogs, cats and foxes.
Cleaning
You will need to clean then out regularly, at least once a week. Sometimes just removing the toilet area midweek will keep the hutch free of smells. Clean the cage floor with a mild detergent or disinfectant, dry thoroughly before putting in fresh sawdust and hay or straw. Always make the bedding deep so that they can snuggle up.
Handling
Make sure you handle your Guinea Pig everyday to make them used to being handled, they are easier to handle than some Rabbits. Place your hand to the front whilst it's in the hutch then scoop it from behind it's bottom with your other hand, securely but not tightly. Make sure they are used to being handled so if they need a Vets visit it will be easier and they will be quite compliant. They will make a squeaky noise when you first start handling them, this will soon stop when they are used to you. But they are quite adept at avoiding capture. Just be confident with them.
Health
They usually have few health problems but one you must look out for are what their droppings are like? Especially in the case of older boars. They are susceptible to a condition know as rectal impaction when the muscle contractions necessary to force their droppings out become weak and they get impacted and tend to accumulate. If you think this might be happening take them to the vet for treatment and a dietary supplement. You can treat this yourself but it isn't pleasant. Just turn your Guinea pig over and pour olive oil on their bottom and massage the droppings out. Of course make sure you are wearing rubber gloves. If you visit the Vets always get a health check done at the same time. Most Vets will do this automatically for you.
Garden Run
Make sure you have a selection of toys and tubes, maybe drain piping to run through in their run along with any obstacles that they can run about in and hide. Be imaginative with their play areas. If you have two Guinea Pigs or a rabbit and Guinea Pig they will chase each other, this is very good exercise for both of them.
Guinea Pigs