Hi xxjon7
To do keyword research you can use the adwords keyword tool. Make sure you localize it to the country you want to target eg, US. Make sure "related searches" and "exact match" (this field will appear in you side navigation once you do the first search) are ticked.
Try to go for keywords with more then 2900 local searches. Check the competition on first page of google only - the top 5. These are the guys you are competing with, don't worry about the rest. If they are strong sites with good on page and off page seo don't compete.
Try and buy a matching domain name with your keywords. If not available add an ending like "review" or "info" but make sure the keyword is at the beginning.
You can also use paid software like Micro Niche finder and Market Samurai to help you.
I came across a site by a guy called Jon Benperthy who has free tutorials on how to do keyword research using adwords tool. Go to jonpenberthy.com and sign up for the free video tutorial.
That's great you that you are building around stuff you are into, this is where you stand to make the most money. That's one of the biggest secrets that most newbies don't realise. Try to go even more niche eg, "natural" bodybuilding supplements.
Go to forums for gamers and body builders and make friends, build a network. Don't sell, just make friends and offer value. After a few weeks you will have a lot of business from your signature link. Because you are into these things it will show-people will trust/like you and potentially buy from you. DON'T SELL to them, at most "recommend" it...
Do this only if you are knowledgeable about your subject otherwise you will not last long as people who are will see through it.
I hope this helps,
Roman