Hi. this is a very good site for tutorials. If you want to learn twitter just go down the right hand side of the page and under the 'Social Media' heading you will see 'Twitter101' these are really excellent tutorials and everything there is completely free.http://www.gcflearnfree.org/topics
However I should point out (althrough I am sure that we all already know) that everything on the interent is constantly being changed all the time, and this is no exception.
Anyway I hope that you enjoy it, regards, Jim T.