For travel documents and visa requirements you can check a very detailed web site of the Ministry of Foreign Affairs of the Republic of Serbia and find all the information including a contact list of all the Serbian embassies and all the embassies in Serbia.
WMO will provide you with a Letter of Invitation.
Note that Republic of Serbia is NOT a member of the European Union but citizens of European Union do not need a visa to enter Serbia, nor do citizens of the USA, Canada, UK, Russian Federation, Peoples Republic of China, Australia, Japan, India and many other countries. Citizens of some countries can enter Serbia if they have a valid multi-entry Schengen (EU) Visa, UK Visa or USA Visa.
You can reach Belgrade by plane, train, bus, car or even boat by Danube river. Largest air companies and few low-cost companies are flying to Nikola Tesla Airport in Belgrade. The national air company is AirSerbia (www.airserbia.com) but you can fly in by low-cost companies such as www.wizzair.com and www.easyjet.com. The full list of air companies that are flying directly to and from Belgrade is on this link:http://www.beg.aero/en/strana/9851/Редован-саобраћај AIRPORTS:
www.beg.aero (Nikola Tesla Airport in Belgrade, Serbia) Nearby airports:
www.nis-airport.com (Constantin the Great Airport in the city of Niš, Serbia 220km south of Belgrade)
www.aerotim.ro (Traian Vuia International Airport in Timișoara, Romania 180km east of Belgrade)
www.bud.hu (Ferenc Liszt International Airport in Budapest, Hungary 370km north of Belgrade)
www.sofia-airport.bg (Sofia Airport in Sofia, Bulgaria 420km south-east of Belgrade) Traveling by train:
www.serbianrailways.com (JSC Serbian Railways)