Is ekg a app that I have to by before I can see my ekg

No, but A) it MUST be allowed by the government in your country, and B) you must have an Apple Watch model that supports ECG.

No, you do not have to buy the app. Just follow the directions in the link above your post. If you are having problems, let us know which Apple Watch you have, what its Watch OS is, and what you have tried so far.