I need a thesis statement for an argumentative essay on internet addiction and being a very serious problem.

A good thesis for this ... is to simply address the problem. For example, you could say, "The internet is becoming an addiction to many people ... " or you could say, "The internet can become a serious problem for many people because of its addiction.". These are a few examples. Be sure to make it fairly clear to what your whole report or essay will be about.