A beginner kit might be a great idea to start, but there's no reason why you couldn't work up to the Mother Theresa pattern fairly quickly. There's a great section on this site of beginner kits that you can find here: http://www.everythingcrossstitch.com/someone-special-mrl-s418.aspx?k2=y4
Or just click on Cross Stitch Kits in the menu and you'll see the For Beginners section. There are some very cute items in there.
That said, I was someone who pretty much jumped in the deep end. After doing really basic cross stitch on gingham fabric as a kid, then doing some stamped projects as a pre-teen, my very first counted cross stitch project was Teresa Wentzler's Rapunzel! It took me years! I almost gave up, haha, but learned a lot working on it and turned cross stitch into a huge passion of mine. So whatever you decide, just make sure you're comfortable with it. :)