This depends.


Brand new activation - Online
T-Mobile.com's Sidekick shop page

Brand new activation - Retail store or 1-800-TMOBILE
Varies. Contact 1-800-TMOBILE or visit the Store Locator to find a nearby store.

Add-on activation - Existing account with a new line
This really depends on how you add the line, and it is too complicated to detail here. (Store, 1-800-TMOBILE, and online in My T-Mobile, basically. NOT T-Mobile.com, as you have an account and the main website will only set up brand new accounts.)

Upgrade - Existing account (except prepaid)
Okay, now this is where it gets a little hairy. With a handset upgrade there are a variety of factors that influence your upgrade pricing, and they can be rather confusing. They depend on your account history, your previous upgrade history, your plan, and your contract, and the results are different for every account. I'll lay out the basics here, but for a definite answer talk to Customer Care at 611.

• At full discounts, you can take a two-year or one-year contract extension, or no contract (full price) -- two-year pricing is better than one-year discounts, but your contract is of course longer
  
• Your plan must be a minimum of $29.99 per month
• You do not get full discount pricing until 11 (22 on a two-year) months of active service after your last discounted upgrade, including when you started
• You may qualify for a discounted upgrade before you hit full discounts, but the prices will not be as good and rebates will not be available (this can mean up to $100 more on the Sidekick 3)
• If you do an upgrade before you hit full discount, your wait starts over again (11 or 22 more months) AND you might not get a discount again
• Exception: If you do an upgrade with no contract extension and buy a phone at full price, it does not reset the counter for the number of months until you get your full price.
• If your account is suspended for non-payment, that entire month is disqualified from counting towards your 11 or 22 months for best prices

So...that's really confusing. It's easiest to just ask Customer Care for what you qualify for and when you will get full discounts if you don't already.