Went to visit a friend there about 8 years ago and travelled around for a few weeks.
It is stunningly beautiful, but you need to be up for an adventure.
Practically - be up for having a wee in the open (no trees in a lot of places!!) or in a long drop loo. Take wet wipes to clean your hands (obviously dispose of them properly). I find wee-ing wearing a skirt gives you more privacy....
There aren't really any roads, so be up for lots of bumpy travel to get around. I hope no one's car sick!
You'll probably spend at least one night in a ger (yurt) which is lovely, but it does mean communal sleeping so take ear plugs in case anyone snores and a torch for finding your way to the outside loo in the dark.
Food can be basic and I hope you like lamb! Almost every meal was sheep-based. Take snacks and an open mind. Mongolian people are amazingly generous, but their palate might be quite different to what you are used to: the fat is a delicacy.
Depending on the level of your trip you might have limited access to modern comforts, such as electricity, so think about that with regard to charging cameras etc. Take some books/games for the evenings as you might be away from modern entertainments.
I was there in Sept and there was a lot of glorious sunshine, but it got chilly at night, so take some light, warm layers to wrap up in. I'd pack in a rucksack rather than a suitcase for easy of moving around.
Are you on a tour or totally independent? Either is possible, but if you are planning on hiring your own transport (there really isn't much if any public transport) then you may find language skills a barrier.
I loved it and if you have an adventurous spirit you'll have an amazing time. Feel free to PM me if you want to ask anything!