I can't speak on every browser, since there are just so many out there, but I’ll give you some instructions on two of the most popular, IE (Internet Explorer) and Firefox.
Firefox - go to Tools --> Options and a box should appear, look right up the top of it and a field called "Home page" and a line called "Location(s)" which is where you type the URL of your homepage in, in this case www.fishforums.net. Click 'ok' to confirm this.
A Great feature of Firefox is that you can launch multiple homepages, so say you regularly browse this forum and say, Google news, it's possible to launch both pages at start up.
IE - Internet explorer is practically the same just go to Tools --> Internet Options and again, it's right up the top of the box which should appear. Input www.fishforums.net and click 'ok' to confirm it.
Like I said, I can't give details on every single browser out there, but in 99% of them, it should very similar to this.