It takes time, dedication, and flexibility. You have to have a lot of things in mind before you can even open the place. What kind of website are you going to be? PG, PG13, etc? Is it going to be Hogwarts only, Hogwarts focused, or are adults welcome. Biography restrictions, do you accept darn near everyone, or are you particular? Advertisements, staff, site wide plots, balancing admin power and user involvement.
A lot of people want to create a website so that they, themselves, can do what they want and when they want IC. ( not saying you are, just saying ) You have to realize that creating a website is more about the members than yourself. You have to be available for every single member, or have staff who can be available. There are so many things that go into creating, running, and maintaining a website that it can be very overbearing at first.
It's easy at first, sure, but when you start getting arguments, complaints, an influx of new characters, blah blah blah, it turns into more of a job than a fun roleaying experience. I know from past experience how difficult jump starting a website is. I've been a founder of various website and only a few have taken off. I mostly advise the staff, help draw a member base, and never admin.
I HATE adminning. It's my least favorite thing in the world. People change around you, they'll hate you, they'll suck up to you, you'll never get a peaceful moment ( On a successfull website ) If you are prone to getting stressed easily then you won't enjoy running your own website.
Some good starter tips I can offer. Get your plot done FIRST. Know what kind of site you're going to be. Pre potter, post potter, AU potter, or plain ol potter era. Know your main story before you do anything else. Then get an advert ready for drawing members. In my experience, if you don't have at least 5-10 members to jump start you aren't going to go anywhere. It's hard to recruit people to a website when there is no one else there to write with them. Also, make sure YOU are available for RP. All the time. You can't have a good website without your self or your team being readily available at all hours.
Also I recommend getting admins from most time zones. Someone who can be available for all time zones so that everyone is represented. There is nothing worse than having to wait for your bio to get checked.
After you get you base set, then you start building. Rules, what layout you're going to have, graphics, titles, positions, jobs, teachers, all that. That's your second stage. Set the pace for your look, feel, and overall meat you're going to have on the website. Is it plot oriented, stay oriented, user oriented, etc.
The biggest mistakes I see on new forums is favoritism. I know myself personally cannot stand cliques. People who only RP with certain people or there are only five people holding all the power positions. That's my biggest pet peeve is when one person has more than one powerful position on a website. As a member your strive to get one of those positions and how is it fair when an admin is Headmaster, Minister of Magic, Warden of Azlabam, you see where I am going.
In the end, you need to grasp the idea that running a forum IS NOT EASY. It isn't about having a place to just go and RP, to be successfull you have to dedicate a large chunk of your daily schedule to it. Don't be deterred, though, because success is awesome and there is nothing like seeing your hard work flourish into a bustlin and beautiful role playing site.
Good luck and I hope that helped!