Welcome Blossom,
(I moved this thread to the Brainstorming forum)
I couldn't agree more with what GreenThumb said. Your first goal is to build an information site about flowers instead of launching your forum first. I made that mistake years ago and never got my forum off the ground because I didn't have the traffic levels to keep it active.
Google can be your best friend and greatest source of traffic if you do it right. Your first step should be to create a site on tropical fish and write a ton of content on it. As you do this, you will begin to be found for numerous topics and keywords related to tropical fish and your traffic will slowly increase over time. It will take months, but as long as you keep building content it will eventually increase.
You can also start a blog at
www.blogger.com for free. This can be in addition to your regular website. You can use blog to post about updates to your site or to write about topics that are not covered on your site. Having a blog point back to your main site will also help in traffic.
I know your main questions revolved around how much it costs and how to build a site but it's important you plan out your goals before you even do this. So many people launch a forum, online store, etc. without a basic understanding for how websites generate traffic. So that's why it's very important no matter what you plan to do, you first build a content-rich website about fish. Once you have enough targeted traffic, making the money (monetization) is actually the easier part.
I would agree with what GreenThumb told you about the costs and building tools. Definitely get a good edtior like Dreamweaver. CoffeeCup.com is good to if you would like to save a bit of money. Dreamweaver is quite expensive ($350-$400) while CoffeeCup is about $40. Obviously Dreamweaver is a lot more powerful but CoffeeCup will get the job done and it has nice design templates already done for you so you just pick a template you want and add text.