I agree with GrumpyGramma, find a pattern that you like and buy the size of needles it recommends. Ravelry is a fantastic sight for patterns.
I also like Knitpicks.com. http://www.knitpicks.com/knitting.cfm
They are my main sight for supplies, needles, yarn, and patterns. They have a good amount of tutorials also.
I love the way they have it set up to choose yarn. Since its for a young person I would recommend sticking with something machine washable.
They have cotton, which Ive recently started using and like very much. I think it feels a bit better than acrylic. Acrylic can be scratchy or very soft, just depends on the brand.
They also have a good amount of mixed yarns.
Im newish myself and am still learning about yarns and mixes.
Needles, they have a set I like. Its a beginner’s set at a very decent price. You get one set of wood needles, one of nickel, one acrylic. That way you can see which one you like best. With nickle the yarn generally comes off the needles faster, wood and acrylic have different grips. Which can come in handy at times.
With circular needles, the cable needs to be smaller than the project, or it wont work out well.
Try it Needles Set
Welcome to knitting! Good luck with your project. Im sure your sister
will love some leg warmers, and they will be unique.
I know at 1st knitting and materials can get confusing. There are so many things you can make, and the learning can be fun!