Hi!
You may find this thread here helpful.
Recommendations For A Balanced General Guinea Pig Diet
It lists in detail what to feed of all food groups. Please be aware that the extra needs of youngs guinea pigs are in fact very small, so don't throw the kitchen sink at them! They will get enough extra calcium and protein simply by a larger amount of pellets (any brand will do at the moment; the majority of them have pretty similar percentages when you look at them) than you give older guinea pigs. Any babies/young adopted guinea pigs of mine have made it just fine on a normal adult diet as we recommend, as they get all the essential nutrients.
You may also find this link here informative; it contains a selection of information threads on the kind of subjects that get most asked for by new owners, so we have bundled them together to make access easier:
" Starter Kit" Of Information Threads For New Owners