Hi redgirl,
I think what you may be looking for is a mattress protector which is an important part of a sleeping system and is meant to have the least possible effect on the feel and performance of a mattress and protect a mattress from body oils and fluids that we release each night as well as spills and accidents. They will also protect the warranty because most warranties are voided with even a small stain on a mattress. Because they are easy to remove and clean they are an important part of keeping your sleeping surface in a clean and hygienic condition.
Mattress pads on the other hand are thicker and while some of them are also waterproof and can double up as a mattress protector, they are meant more to affect and change the feel of the mattress more than a mattress protector.
There is more about the differences between a mattress protector and a mattress cover in post #5 here and there is also more about the tradeoffs involved with different types of mattress protectors and some good sources for each of them in post #89 here.
Unless you want to change the feel of your mattress with a mattress pad … if your mattress is already a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) then a mattress protector that is the best match for the criteria that are most important to you and has the least possible effect on your mattress is probably the best choice.
And congratulations on your new mattress 
Phoenix
NOTE ADDED: They are now a member of this site as well