Every lj user has an open id account so you just need to login at DW. Detailed instructions are here.
Also make sure to verify/validate your email
Invite codes for full DW accounts (that allow you to post entires, import your lj with a cllick of a button etc.) are being sent randomly to anyone with a open id account validated at DW. If you get an invite before Open Beta on the 30th, here's what you can do in closed beta.
At Open Beta you can otherwise get a paid account (as explained) and skip the invite code entirely. The minimum cost without a code at Open Beta is $3 for one month of paid membership. After that month you'll still have the account - it will just revert back to free status.
Payment costs for each level (paid vs premium vs seed) are here while info on what each account level gets is here
If you're not interested in getting an account but would like to follow the posts made by people who move there, here are your options
Three other useful posts: