how to tell people no...

hi. i have a problem when it comes to talking about things to people. i mean, i can do it fine when i run the conversations over in my head, but when it comes time to actually say those things, i become tongue tied and it all comes out wrong and i either say the wrong thing, or sound like a bitch. (i am female).
is there any easy way to get yourself to open up your innermost feelings to people, or is it just something you have to learn????

xoLeah.