Title - the thread title should be "[Vx (build number) - Ground Combat / Air Combat / Geoscape] Bug Name"
ie, "[V7.5 - Ground Combat] Aliens have infinite health!"
The version number is found in the top-right of the game launcher. Please note that if you are playing Xenonauts: Community Edition you should not be reporting bugs here - it is an unofficial project and the dev team cannot provide support for it. The same goes for modded games.
Post - the post should contain details of what you were doing prior to the bug being caused, what the effect of the bug was, and reproduction steps if you've had a chance to find any. Screenshots and save game files can also be attached if you think it'll help us - save games in particular are useful if the game keeps crashing at a certain point (they are found under My Documents and can be attached under the Advanced Reply forum options).
If your bug is technical rather than gameplay-related, please provide details of your PC too.