I'm not sure if you mean to file an official report on the net or to an agency, But if indeed you do mean on the net (Which i am sure you do) then
the links that people have given you should be fine, As long as you dont post Identity etc..
But as far as official sightings off the net goes, I wouldnt advise you to go down that road. I would keep that to yourself in actual life, As
government intakes on civilians reporting UFOs isnt exactly a topic they are happy about, If you know what i mean.
As long as you do not reveal identity over here, Its fine.