Do you think body building would be good?
As others mentioned, you'll want to figure out how to monetize it first.
Body building is a bit too broad for starting out. You need to define a niche.
Maybe your niche is workout plans or body building pills(not even necessarily steroids) or something else.
Then you identify long tail keywords such as: What is the best body building pill
Create a page for that...Have the keyword be the title of the webpage and the h1 title of the article and then just write naturally.
Do research, watch youtube videos, listen to podcasts, read Amazon reviews and find out what the best products are...
At that point if you're monetizing by Amazon you look for related Amazon products and you put them in your list.
Or adsense, target long tail keywords with better cpc.
Your goal for keywords is typically - Low competition, easy to rank, and high search volume...you won't always find this.
Since you're just starting out, look for keywords that are easy to rank, but may not have as much search volume.
There are lots of posts on here on how to do keyword research or you can even google it and find youtube videos.
There are also people here that will do keyword research if you have the cash.
You can do review sites, comparison sites, news sites, or something else.
Write for the end user, not SEO and you'll be fine.
Run it through Word, spell check, and grammar check.
Read it aloud and make sure it makes sense...
Send it to a friend and make sure they can understand it. Is it too high level? are you using too many acronyms?
There are so many variables and things to research... it'll take some time and patience to get the hang of it.
You'll preferably need either some time or money to devote to it.
Maybe Monday and Wednesday nights you say I'm working on my niche site these nights and nothing else.
Don't get burned out or you won't finish...
There are tons of niches and products to sell...as mentioned above...find something you know alot about and enjoy...and then break that down into a niche site to get started.
You can also do an authority site, it requires a lot more time and effort though.