Before you can let Health track data from other apps, you need to have some compatible apps installed. I happen to have a Withings Pulse fitness tracker and Smart Body Analyzer scale, so I installed the companion Withings iPhone app. I also installed the Nike+ Running app so I can collect data from multiple sources.
You'll need to tell each app it's OK to share data with Health, and so far I've seen a couple ways to make this happen. Some apps, like Nike+ Running, will ask when launched if you want to share data with Health. Others, like the Withings app, enable Health sharing through their settings.
Some apps, like Withings, let you enable Health support instead of asking at launch
Once you have at least one app set up for sharing, hop into the Health app and tap Sources. The apps you previously set to share data show up in a list. Just tap an app to choose which data points you want to send to Health.
Health shows with apps can share data via its Sources tab
You can choose what data gets shared with Health from each app
Now tap Health Data and work through the categories to choose exactly what information you want displayed in Health's Dashboard. Once you're finished, tap Dashboard to see all the health and fitness data displayed in a single screen. Depending on how much information you're showing, you may need to scroll down to see everything.
Use the Health Data tab so choose what you see in your Health Dashboard
You can add and remove data points from Health any time you like without worrying about losing information because everything is still safely stored away in the apps that are handling the collection process.
My Health Dashboard shows how far I walk, stairs I climb, and how I sleep
If you aren't sure if you have any Health-compatible apps installed, check out TMO's list. If you want to learn how to set up your own emergency information in the Health app, check out Melissa Holt's awesome tip.