The subscription can be bought via Amazon and it won’t kick in until the service launches. Note that the pre-purchase is only for the 12-month membership which is priced at $20 for essentially the entire year. If you’re unsure if you plan to stick around for the whole year, Nintendo will also be selling subscriptions at $3.99 a month, or $7.99 if buy it in blocks of 3 months, but clearly the annual option is cheaper.
Nintendo’s online service is considerably cheaper than the competition, although we wouldn’t be surprised if in the future they might raise those prices. The online service will be required for gamers to play their games online, so unless you plan on playing single-player games or local co-op games, you will need to get a subscription to play with others online.
The service is currently set for a launch this coming September so you’ve still got a couple of months to think it over.