Every school has a chance of joining the MUN and it's not very hard to start a delegation. The first thing a school would have to do is to contact an MUN conference organization to see if it is possible to join. An MUN organization will never deny the admittance of a new school without valid reasons.
The only possible reasons are that the MUN doesn't have enough facilities to admit more delegations or it does not have anymore countries to assign schools. If the MUN primarily says that you will be able to join then you can pretty much take that as a yes. You will then have to get a few forms from them and register the school with the conference. All you have to do next is to wait for a country to be assigned to your delegation.
Usually, every delegation has the option of submitting a form with a list of countries that they would like to have, but the ultimate choice is up to the MUN. Schools that have been attending the MUN for a long time may get their first choice, but first-time delegations will probably have to do with whatever choices are left over.
Once assigned a country, delegates usually decide which committee they wish to attend, according to their interests. Once this is done the delegation is ready to set of to a great adventure. Try our links section, and find a conference to start with if you haven't found one already.