The main thing that you have to remember for witnessing to Mormons and Jehova's Witnesses, theoddamerican, is that they have different definitions for things than we do. Look up their faith terms, and make sure to steer clear of them, and use different wording. Explain, in simple, non-ecclesiastical language, how Salvation works (Watchman Nee has put together an excellent study of this in his book "The Normal Christian Life") and why we can't get there by selling magazines, etc.
But the first and foremost thing you need to do in witnessing to them is praying. They're a part of a cult, and those things are hard to get out of, both in your mind and in their system. They won't want to change their minds. So, the biggest thing you need to do for them is pray.
Toyjarta, this is not uncommon, so don't feel like the odd one out

But you still should learn about things so that you
will have an answer to the tough questions you'll be asked when witnessing. Learn your stuff, and you won't be left blank when they try to prove to you that God doesn't exist, or isn't who you think He is.
Try books by Lee Strobel, including some that you should never be without:
The Case for Christ (historical accuracy of Bible, therefore idenity of God as Yahweh)
The Case for a Creator (scientific accuracy of Christianity, and Creationism in general)
Then, there's C.S. Lewis. You should read as much of his as you can, but in case you're not a bookie like me (

), here's an excellent work of apologetics:
Mere Christianity (basic case for existence, identity, supremacy and morality of God)
Then, of course, there will be spiritual questions that you will not know the answer to right off the bat. For this, you should really know your Bible as well as you can. Then, pray for answers to passages you don't understand.
In all things, pray for God to direct you to His will, and to help you do it in the way He wants.
Hope this helps

~Lily