Except for the happy exceptions of US-citizens (please check the embassy web site nevertheless), most citizens of the world need a visa to enter Mongolia. But also for the rest of the world it isn´t difficult to get a visa to Mongolia. Generally all it takes, is to fill out a form, pay a fee and you get your thirty days. Should you need an invitation though we would provide you wit it.
But then procedure varies slightly from country to country. Here links to the departments of Mongolian embassies in select countries: