One of your first stops should be Rothiemurchus Estate Visitor Centre. Here, you'll be able to orient yourself and get lots of useful information before you set out exploring.
You'll find this tourist information center in Aviemore. Once you've seen everything you want to at Rothiemurchus Estate Visitor Centre, take a look at nearby Cairngorm Reindeer Centre, Loch Morlich, and Loch an Eilein.