Welcome to knitting!! WalMart doesn't have the nicest yarn on the planet, but much of it has some good qualities. They carry good ole Red Heart acrylic in worsted weight and it will wear forever and has been the workhorse of the yarn world for many years. Red Heart also makes a "Soft" now which is just that, softer than the regular stuff. A lot of people like Caron's Simply Soft. It is very soft and I guess it washes up okay. I believe WalMart sells Vanna's Choice which I also think is pretty good. I have seen some wool at Wallie World, but don't know which kind or if it is always there. It is probably okay wool, if they have it. The store also has basic cotton yarn if you want to knit dishcloths.
My personal advise is to start with one of the acrylics in a smooth (the ones I mentioned above are all smooth) yarn. I don't like cotton for beginners because it doesn't have any give to it and is stiff and a little harder to work with (but some people love to start with dishcloths and swear by it). Stay away from the interesting, but bumpy, hairy or whatever yarns until you have a little practice. Your idea of white is good or the kind of colors you mentioned. What is not good for beginners is something real dark like black or close to black of any color, black being the worst. It is hard to see the stitches with very dark colors.
There are a lot of kinds and qualities of yarn in the world, but in my opinion Wal-Mart isn't a bad place to start. Have fun.