As piggy is still new you need to leave him/her to settle in. Perhaps cover the cage with a blanket. Put the food and hay near his/her hide. They only drink what they need as most water they get from their veg.
Hay makes up the largest part of their diet so make sure there is plenty available. Pellets only a tablespoon a day. And 50g of veg. I will link in some guides below for you to read. If muesli is the only option then it’s best not to feed them that at all
I notice you say a guinea pig. Are you planning to get him/her a friend? They are very social animals and need constant company of their own kind. No amount of you interacting with them will do.
Best thing to do is get a same sex piggy. Cage size is important as well, more so with boys. Boys need minimum 150x60cm (180x60cm would be much better). Girls need minimum 120x60cm (if you can go bigger then all the better).
Do you know how old (s)he is? If under four months then company is even more important.
Lastly

. We love pigtures and to know more about your piggy so do go introduce yourself and start a thread for your boy/girl. In the meantime have a read of the guides linked below.
Arrival in a home from the perspective of pet shop guinea pigs
Long Term Balanced General And Special Needs Guinea Pig Diets
Companionship
How Do I Settle Shy New Guinea Pigs?