What is the legal requirment to use "TM" or "®"?

Under federal trademark law, the "TM" and "SM" give notice that you have filed an application to register a trademark or service mark. (There is no effective difference between the two: a trademark is for goods, and a service mark is for services.)

The ® gives notice that you have actually obtained a registration on your mark. Don't use it until you get the registration.

Giving notice can be important if you ever run into infringement of your mark. If you have given proper notice with these symbols, you can get better damages in court.

More details here:
http://www.inta.org/TrademarkBasics/FactSheets/Pages/MarkingRequirementsFactSheet.aspx

The TM symbol indicates a claim of trademark rights. The marking may indicate that a trademark application is pending or may indicate a common law mark (rights based on actual use and not necessarily pending with the IP office).

The SM symbol indicates a claim of service mark rights (i.e. a pending or common law mark for web services rather than a product).

The ® indicates registered trademark rights, meaning that you have filed a trademark application with the relevant country IP office and subsequently have been granted a registration from that IP office.

Hope this helps.