Adverts...

Hi all,
I understand that we have to have adverts on our forums due to them being free.

Could someone please tel me if there's a way of getting adverts to appear on our sites relevent to our site content?
ie my forum is about fishing so maybe fishing related adverts...

If this can be done would someone be kind enough to tell me how I go about it

Thanks in advance

Lee

Generally those ads appear according to your searches made on google in the past.