When you go on the offical online forums, there is a sub-forum called Sell Order. From there you browse through the various threads where sellers display what they sell.
Mind you the transaction between ISK and GTC is covered by CCP so if you get scammed, you can always appeal (be sure to keep record of who what and where you're buying).
Usually, once you have determined a good seller, you contact him (conversation or eve mail), then send him the isk in exchange for the code which you will activate in your subscription details.
A while ago, the rate was 100million for 30 days and 270million for 90 days. Prices seemed to have jumped a tad since then.