There's a number of ways, you can go to the goodies section of Twitter and they have a variety of widgets or there are plenty of third party services like Addthis that offer the buttons.

AddThis and ShareThis offer that functionality.

Note that it involves a call to their servers for each page that displays their widget. This may add to load times, and you'll be sharing all of your visitor data with a third party.

oh boy which is best and which is eaier, I use wordpress.org

Now it says I don't have enough room, it says

Compatibility
WordPress
+Plugin
=
Not enough data

what do I do now?

I use Addthis on my wordpress installation... you can find it by searching for Addthis in the new plugins section of admin.

I can't help you with the wordpress not enough data problem I'm afraid.

I got it going, thank you!

Now it says I don't have enough room,

What is this "it" you are referring to? Not enough room for what? (Just to add to my knowledge bank,0 as I have not seen that error. :)

There are two kinds of Twitter icons. One is the Tweet This button - used to be Tweetmeme but they are pointing people to Twitter's equivalent. The second is the "Follow Me" kind. You will probably need both.

I've got the follow me kind

I will look at tweetmeme tomorrow, why do I need both please?

the follow me link takes me right to my twitter page, is that correct?