My thesis: internet can be an addiction.
i want to argue this point on 3 topis sentences. why it's an addiction. facebook, psychological effect on our mind,(something similar.) and myspace.
but i don't know how to construct or properly write an argumentative topic sentence

Back up your argumentative sentence with evidence of what you're writing about.
Example: As of 2012, more than half of the human population joined social media according to the scientific study at the university of utah.

You can't write a paper about "everything" because it would never end. Two of your three topics are social networking sites, the other a psychological phenomenon. Your thesis is about addiction. So you should choose whether you are going to talk about social networking addiction, which might be something called "virtual interaction" (there are many names). Virtual identity or "-philia" is probably what you are referring to, unless you specifically mean talking to people over the internet. It is a phenomenon where someone has become addicted to their "virtual" interactions - whatever they may be - because somewhere psychologically they prefer those interactions over physical everyday life. This can be playing games, music, videos, reading blogs, etc.. It doesn't have to actually be social. Remember that addictions are not taken lightly. They are habits that when stopped CAUSE TRAUMA, or habits that INHIBIT WHOLESOME LIVING. That is the only reason they are considered bad, otherwise they would just be avid normal interests. The internet is very useful - we are on it right now - and using it regularly is not innately negative. Some of us use it for livelihoods. So if you mean that the internet is addicting, but not bad - you might merely mean the internet is ENJOYABLE. So if about social networking, you could state why you think everyone using them should be careful. If about deluding oneself from reality, you could state why people form this dependency or what to do if they have it.