Well I would start with openid or oauth(for twitter), I found those the easiest to setup so far.
http://openidenabled.com/
I will warn that I had a little trouble setting it up in a locally so you may want to throw that up on your server to test it.
For twitter you need to create a application on twitter and get the secret key's, And again if your doing this local use 127.0.0.1 twitter doesn't accept localhost.
jaisenmathai.com/blog/2009/03/31/how-to-quickly-integrate-with-twitters-oauth-api-using-php/