You'll need to hurry, though, as Jagex is only allowing new-player registration on the throw-back servers for a limited time as part of the browser title's 10-year anniversary celebration. Make sure you log in between Thursday, September 15th and Thursday, September 29th to ensure that you can access RuneScape Classic in the future, as this is the last time Jagex will be allowing new players to use the service. You'll also want to collect the unique skillcape emote and the classic cape and hood items since they'll be unobtainable after the conclusion of this event.
As always, the RuneScape website has full details.
[Source: Jagex press release]