You should learn the context as well to apply them properly. Try to join a group, find a teacher first. Plus many of the mantras are traditionally restricted to those who got empowerment from the guru (lama). Usually what is said to be ok to use without any additional permission (besides some generic ones) is the OM MANI PADME HUNG mantra of Avalokiteshvara and the mantra of Green Tara. Some teachers would say there are even more. You can even listen to Tsem Rinpoche speaking about this topic in the video called: "Which Practices Need Initiation and Which Don’t"
