There were no "good guys" in the Great War. Every participating country had reasons to wage war, determining who was good and who was bad is practically impossible.
Basically the Austro-Hungarian Empire started the war, but it means nothing, since the war was inevitable.
If you mean that good guys = winners, then :
British Empire
French Republic
Russian Empire
United States
Italy
Japan
Serbia
Romania
Belgium
Brazil
Montenegro
Australia
Canada
Greece
India
Nepal
New Zealand
Portugal
South Africa
There may be a few other minor participants, but I don't think you actually need them all, it's not really important to know all of them.
