If you are a new user on SteemIt, your Steem Power will be 0.
Posting is the only way to Increase your Steem Power. While posting you will get a option to select how much power you need for that post (0%, 50% or 100%).
After 7 days of your join, you will get your first payment. At that time your Steem Power will Increase.
In the second week your upvotes are more powerful than your first week.
Hope you understand. Keep enjoying #SteemIt.