Moderating an online forum require vigilance and believe it or not, some people skills. Here are some tips on how to go about this.
Read the rules and regulations of the forum. Make sure you know them and understand them. Moderating forums is not about you or your opinion but the forum as a whole. This will keep you from being all too lord-and-mighty. Be fair.
Get acquainted with the members of the forum. This might be impossible if there are hundreds of members, but try to know the active posters or those who lurk around.
Be active as well. Members of forums would like to know that the moderator is a human and not a robot.