As lws said, the main thing is to start racking up the points so you can get to Gold as quickly as possible, to maximise the discount on an Emirates flight to the USA (and all of the other Vitality benefits!)
The first thing to do is book a Health Check at a Clicks or Dischem. You can earn 20,000pts in one go if all of your numbers are in the correct range. And you need to have done a Health Check plus an online "Vitality Age" assessment on the website, in order to get the full 25% HealthyFood discount at either PnP or Woolworths (you choose which one). You will have to get a HealthyFood card, which you swipe at the till. Then pay for your food with cash or your FNB credit card as usual. Your 25% cashback will be paid into your bank account once a month. And you can earn up to 1000 points per month - 20 pts per item of Healthy Food, but you lose 20 points for every unhealthy item.
Go to the Discovery website, click on Vitality at the top and then Track and Earn points. Go through the page carefully and it will tell you all the things you can do to earn points.
You need to make the flight booking at least 6 weeks before your date of travel. But even if you haven't achieved Gold by then, you should at least be Silver, which is still a 25% discount on the base fare of the flight. Gold would get you 30%.
Just to give you an idea - I'm already on 36000 points, just from my Health Check and fitness activities.