Basically what I mean is that you need to create an account and start posting information as well as retweeting information you find interesting. You should also start following people you find interesting and that pertain to your interests and your career field.
What you will found is that if you follow someone or retweet their post they will tend to start following you. People on Twitter are really that nice. :) It may seem a little odd but true.
The other question is - what to post. This is really up to you. Sometimes there is no rhyme or reason why certain posts are more popular than others. Basically my philosophy is to post things you would normally want to tell your friends or colleagues. That is what Twitter is about - posting short pieces of news to your friends and colleagues via the WORLD.
Twitter makes things really easy to post almost anything. So if you read an interesting blog you can post the link. If you see take an awesome photo you can attach it to you post and so on. There are really almost limitless possibilities.
However, I will give you a few words of advice. One don't post everything all at once - meaning don't log in just once a day and post everything you want to say at that time. Try and spread your posts throughout the day. Also, be kind and retweet. This is kind of a general twitter rule and thank those that have retweeted your posts. Kindness really does pay off in the Twitosphere.
Other than that just have fun and enjoy all the information you will be able to glean from the world we call Twitter!