To protect your company's domain, you can file a dispute with ICANN. I don't think it's necessary to register your trademark first, but having a registered trademark does help in the dispute proceedings. If you do need to register, hire a lawyer to do this.

If it were me, depending on the circumstances, I would probably wait to register a federal trademark until a Series A round, since you have other things to worry about before then.

Disclaimer: This is not a legal opinion, just a personal one.