A registered trademark symbol is different from a normal trademark symbol. In the US, the registered symbol is easily recognized as a capital R with a circle around it.
The way to create a registered trademark symbol in html is to use the ® entitye code - which looks like this: ® . Just like a normal trademark symbol, you can use this designation after your logo or brand name to alerty others that the mark is used for your trade and should not be copied. However, there are restrictions to the use of the registered version of the symbol.
The difference between the registered symbol and a normal symbol is that the registered symbol must have been filed with and accepted by the us patent and trademark office as your official trademark. This means that the registered trademark symbol actually holds a lot more weight than the normal one, because the US government has researched and then granted the exclusive use of the mark to trademark holder. This also means the mark is valid and recognized across the entire US, versus the normal TM mark which implicitly means that the mark exclusvity only applies in areas of the US where trade is being performed. This could mean your mark is valid in one state, but the next state over it could be legal for another entity to use your same mark!
To register a trademark with the USPTO, normally an IP lawyer is used to prepare and file the request. The office will charge around $300 for the filing, but your legal work will likely cost more. It is possible to file a trademark as an individual as well, but it can be tricky... so only attempt this if you've read up on the process or already have a handle on how it is done.
Again, to use a registered trademark symbol on the web, use the ® entity, which creates a ®.