It depends on what you try to promote.
Each niche/product/service usually has one or more channels that are specific for it.
For example, for somethings a website is crucial. For others might be enough to promote it on social media or forums for example.
However, no matter the channel, you want to build your authority.
Lets say you want to sale backlinks on BHW. You pay for the account upgrade, make a nice bts, get approved, pay for BTS.
Do you think there will be many people to buy from you if you have 2 posts on your account?
Or do you think people will tend to buy from people they've seen around, from people that offer value to the community?
Sure, you can fake many things. But, IMO and experience, in the end it is cheaper to be genuine, offer some value, get your 'face' out there and clients will come.
So you can approach affiliate marketing in at least 2 ways:
1. start with the product/service that you want to promote. And then figure out where and how to promote that.
2. you can start with things that you are good at. Build some authority (it can be a blog/website, social media, forum). At some point, even if you don't have the intention to monetize it, the opportunities will present themselves if you offer value to your community.