Get the nearest lawyer around you if you want to file one. But before doing so, check the registrar TOS, confirm that you are not the one in the wrong before doing that. Also bear in mind litigation costs can be expensive.
You need to file with WIPO. I've done this a couple of times and won every case I've litigated. However, you may have no basis for a lawsuit. For example, if you registered cocacola.com and was selling soft drinks, what would be your defense? You've violated law and the registrar's terms of use.
Nailed it. And if some of you folks are curious to know if a registrar's contract has been upheld, here's one answer: http://davezan.com/sizevsnsi.txt Read counts II and III of that decision. If that's the registrar in question, OP, then your chances of winning are practically nil. Otherwise, do what you see fit.
Start from step one. Why was the domain ownership terminated? We don't even know that yet and it is the key to all the answers. No need to do any of this stuff until this question is answered.