You only have direct control of your own character. Your character may have a spouse, a family member, or even a pet, over which only you have control, but that is about it. You may not control the actions of other characters.
Stick to writing about what your character knows and is aware of.
As roleplaying is part writing, part reading, part gaming, and part using your imagination, it is important that your posts be legible and easy to use. Try to avoid netspeak, leetspeak and short hand writing. Try to seperate dialogue and action. Posting in third person and using past tense tends to help.
If you are going to write something out of character, then please mention it as an OOC comment. The best way to do this, is to put the OOC comment between the OOC tags in the post.