First off invoking a deity is normally done in every formal ritual. Its part of the casting the circle after calling the elements. One invokes the presence of deity, not summons it literally. When you pray to your deity, you are in a sense invoking it to come and listen to your prayer...so it isn't anything major really. You will never actually see your deity except in signs given to you, or while in meditation and dreams states. The most you will get is a sensation that she is with you.
And Mother Nature is a very general concept. Have you considered looking into Goddesses from different mythologies to see if one fits your idea of Mother Nature? To many the names are all different, but the deity behind them is the same. So in this manner, all Goddesses that embody the Earth and nature would all really be the same goddess who has just been given different names and faces over time. Many find it easier though to connect to their personal deities if they have a specific name to call on. But not everyone of course. Always do what you feel is best on this matter. Its just an idea. :)
One last thing though, I don't recommend you start doing rituals any time soon. You need to take the time to study and learn more about magick before trying anything. There is a lot one needs to understand before doing any magickal undertakings. Like altar setups, tools, color properties, how to cast a circle, understanding the elements, why magick works to begin with and how....etc. There is a lot to learn to do a successful ritual or spell. Try your local library for books on witchcraft, paganism and wicca. Many of them have books available on these topics. Online isn't always the greatest source because of all the misinformation out there on witchcraft. But www.witchcraft.org and www.witchschool.com are some good ones. The second one is actually an online lesson site that hosts from free lessons you might enjoy.
Good luck on your magickal journey!