I know this was some time ago, just thought I'd reply in case it's helpful to someone.
If you don't want to take a tour, you can take a bus there (just search the forum for the latest details). Whether or not a reservation has to be made depends on which ger camp you want to stay in (big, nicer facilities, nearer the road, meals, etc.), when you're going and how willing you are to wander around asking.
Outside of the tourist season you don't usually have to make reservations, but even if you turn up in the peak season in summer and don't get a place in the first camp you go to, you can usually ask around and find a place elsewhere. However, if you aren't bringing any supplies with you it's generally better to give advance notice -- the ger camp may not have any food there if you haven't told them you want meals.