Learn about the topic, there is a ton of information on any niche out there (both free and paid).
One key that I learned from reading Tim Ferriss' book (The Four Hour Work Week) is that you do not need to be an expert... you just need to be the expert to your target audience.
This is a horrible example, but it fits...
If you are teaching people how to create and host a wordpress blog, your target audience is obviously people who do not know how, you just need to know enough to tell them how to do it.
Another example: I have been diving headlong into affiliate marketing (working with article marketing and
SEO). At my J.O.B. I am the expert in affiliate marketing because nobody else there does it, or even knows what it is. To them I am an expert because I can show them my clickbank/amazon/google accounts and show them a cash balance that is mine... even though they do not know that I am not near payoff levels (well on amazon and clickbank I am... got a long while until google
)
Another key from Ferriss.
It is really easy to become the expert. You just need to apply yourself. Read the top 3 books on your topic and you will know more than quite a few people.. and the more you learn beyond that just further separates you from others.