How to change the default template for left menu or template to build-in?

Hi, i’am a french newbie by Modx and i’ve been trying to change first the default template or to build in a new one.
I wanna one left menu and an upper menu and in the center the content displayed. Only that for the moment. I could change myself the default CSS.
Where can I find new simple template to download. Thanks for support.

There’s a ton of them in the Repository (link in the upper right of your screen). I’ve got a few on my loripsum.co.uk site as well (link in my sig below).