Hi Sue, I'm just back from the tour, it was fantastic.Not a lot of time to see things not on the itinery as you are kept so busy seeing things. Some days are just traveling but your driver will stop to let you see things & take photos. Clothes should be very casual as they dont "dress" for dinner, in fact we did'nt get time to change some nights. Take things you can "layer" as the weather we had was mixed, we even had a little snow in Yellowstone.In Feb you will need warm things as you will certainly have snow in Yellowstone, it should be fantastic then.
Dont worry about it being your first time on your own, you will not be the only one who is a first timer, you will be well looked after.I have just done my 4th trip & I still feel nervous, what I found helpfull this time was going to the Pre-tour get together. You will know some people right at the start & it won't seem so daunting. Have fun.
Regards Sheila