Can I just make a Windows game and sell it on Steam ?

Probably obvious question ,just had to ask lol ^^

Probably yes. Now if you make a lot of money out of it, there are restriction for next year if you keep on developing with Unity.

Check the EULA, it is all there. Unity Software License Agreement 4.x

Unity does not get involve in the process of submission to Steam or other platforms.

Note that asking a public forum about legal questions is not a good idea. There are few representatives of Unity with the legal rights around here and responsibility is all yours if I tell you wrong.