One of the common misconceptions in learning a foreign language is that a native speaker makes the best teacher of that language. While a native speaker can be an excellent teacher, the mere qualification of speaking that language does not qualify one to teach it.
Think of it this way: you're reading this post, so presumably you speak English. You are probably even a native speaker of it. Now think about someone who does not speak English. If your task were to teach that person English, what would you do? Where would you even begin? How would you assign meaning to a jumble of words? How would you introduce grammatical concepts in a meaningful way? Do you even know the grammatical rules and terminology that you would need? In what sequence would you conduct your lesson?
Unless you are a linguaphile, you probably would have no idea what to do. Even if you did, would you know how to effectively convey that information to your student? The answer for most people is no, because the mere fact that one speaks a language does not qualify one to teach it.
Teaching is a skill. One must acquire a solid understanding of student learning styles and teaching methods, based both on theory and practice. A good teacher understands the difference between student-centered vs. teacher-centered learning, both in theory and in practice, and utilizes the former, not the latter. A good teacher understands different student learning styles - is the student a visual, audio or kinesthetic learner? Not only does a good teacher understand the difference, but s/he can identify right away with which style the student best learns and adjust his/her teaching style to optimize the learning experience for the student. A good teacher knows ahead of time if the way s/he is presenting the material will be comprehensible and meaningful to the student, and will utilize the best method for that student.
Language teaching is an even finer-honed art. Language acquisition theories vary widely and are different than general teaching/learning theories. A good language teacher is familiar with these language acquisition theories, and understands through case studies which approaches are more and less successful. A good language teacher utilizes this theoretical knowledge to optimize the learning experience for the student, and knows first-handed which approaches work best for which type of student. A good language teacher will construct a communicative context for the student to learn, practice and apply his/her knowledge in a useful and meaningful manner. Only a thoughtful, intuitive and TRAINED teacher can provide this.
If you are considering learning a language, do yourself a favor: choose a trained teacher of that language. The truth is that it doesn't matter if the teacher is a native speaker of the language - if the teacher is trained, s/he is an expert in that language and will be 500% better than any teacher whose qualification for teaching that language is the fact that they speak it.
P.S. - How is your English? Do you know the correct grammatical rules and terminology? Do you speak correct English, or English correctly (and what is the linguistic difference between those two?)?