Hacked

I have had my site hacked so many times of late.
I did not set the site up as I don't know a lot about php. I am a cyclist and the site is for my club.
Three days ago it was hacked by a person called reaver@darkmindz.com and they put a link to that site. I had a quick look at the site and it appears to be a bunch of people dedicated to hacking sites. I also noticed the owner of that site is a member here (Darkmindz).
Any way I did not have much time so I left my site as it was because they only changed the first page and left the content alone. However, today when I when to the site it has been totally f***ed up. The language has been changed and I can not log in as an administrator and when I look at the number of members it appears that 4 accounts have been deleted and this I think corresponds to the number of Admins there were on the site. Thisd time the person left the following message everywhere "Hacked by Cre3p".
Very annoying.
It also appears all the content is gone. This is ****eful as we are a volunteer organisation and these people are so childish.
Oh I'm starting to rant.
Does anyone know how I can get control of the site again.