If the webdesigner and the administrator of your TYPO3 site have configured everything properly, it's really easy to choose a different template for your page. Just create a new page (or use an existing one) and choose the type "Advanced":

Scroll down the entire form and on the bottom you'll find the template selector. It will show all templates that are available for your website (of course your webdesigner and TYPO3-admin will have to create them first). Depending on your website you may choose between a main template and a template for the content area:

Just make your choice and save the modified page header. Depending on the template you have chosen you now might have more or less columns available for filling with content. Please ask your webdesigner for your website's layout details.