Yes, it is true Russia joined WW1 because of a previous agreements to protect Serbia
serbia
Serbia i think
Serbia
Serbia and austria-Hungary
Allies ... use wikipedia for more information.
Russia helped Serbia when Serbia had rebelled
Yes. After the years of the death of Tito, it began to collapse and formed into Serbia and Montenegro. By 2006, it had formed into two different countries: Serbia, and Montenegro.
Austria-Hungary declares war on Serbia.
Russia wanted to protect the Slavic peoples of Serbia
Serbia pre WW1
It wasn't directly a country that started WW1 but there wasn't a specific event that caused it either, it was just a build up of events. One of the most common and one of the biggest events that happened before WW1 was the assassination of Arch Duke Franz Ferdinand.