adding theme to wordpress

I have a wordpress account and blog already and I didn’t make the instellation, but a friend. I want to change themes from www.remarpro.com, I know I have to downlaod the theme and follow the instructions and then get softwear to put it on the web, Is this right? Do I have to get a hole new account and start from scratch? The existing wordpress I am on hasn’t got the theme I want,
thank u,
Ilaria