Terro's advice is good, but as you will know if this is your third attempt, running and being responsible for a forum is a very hard thing to do.
If you do focus your forum try not to make it too narrow a focus. If you specialise too much you could grow old waiting for enough people who would be interested enough to join to even find your forum.
If you start an FAQ try to make sure that the information you give is correct to the best of your knowledge. Nothing closes a forum faster than potential members believing that you are a waste of their time.
The last thing I will say is the longest but is, I think, the most important. I found that finding the right balance between giving members the right to free speech, and not allowing some, more belligerent and vocal members to intimidate other members so that they don't post is one of the hardest things to accomplish.
As Terro said 'you don't want to run a tyranny', if you are too strict then some people will not join and your forum may die a natural death, but on the other hand, if you are not strict enough then you risk allowing some members to become so forceful and unruly that they intimidate and so subdue the other members.
The trick is to get to know your members, watch their behaviour and read their posts. Keep encouraging the more shy and quieter ones, and use a firm hand with the over-forceful ones, in order to keep your forum a happy and welcoming place.
It is unfortunate but inevitable that the forceful members will resent the fact that they are not allowed to say all the things they wish to say, and may accuse you of being dictatorial. They will try and shame you into letting them get away with being rude and pushy by saying that you are denying them their right to the free speech you promised them when they joined. However it is important to stick to your guns with these people. Their right to free speech does not give them the right to bully others, nor does it give them the right to make your forum appear mean, argumentative, bigoted, unwelcoming, or intimidating. A forum that allows this kind of behaviour does not encourage new membership, and ultimately dies.
My suggestion would be that you offer your membership free speech up to the point where they overstep the boundaries of politeness, consideration and well mannered behaviour towards others that you have set. Remember that even though you run your forum for the members it is
your forum, and you cannot allow one or two members to potentially spoil it for all the rest.
I really don't mean to offend you by appearing to suggest that you cannot handle yourself, but I wanted to warn you about this possibility. I have had this kind of trouble, but I allowed it to get out of hand. In the end it wasn't worth the hassle.
Perhaps I could have tried harder, but I had other things going on in my life at the time, and they had to take priority.
I hate to bring my past into my present, especially to put it out in a discouraging way, it just felt like an important lesson to pass on to anyone who is thinking of opening a forum in the future.
I wish you a lot of luck with your forum. I hope, like the saying goes, the third time is lucky for you.