Forming an LLP (not an LLC - that is an American company) or an LTD limits your liability. What this means is that should something happen to your business, for example you get sued in the courts, then your liability is limited to the money that you have invested in the business. This cannot be said of a standard partnership with a friend/associate or a soletrader business. In these cases it would be you personally that is persued in court not the company or LLP.
I appreciate that I have given a basic example but the point is to demonstrate how you can protect your liability. However, my earlier post suggested that you may not need this kind of protection. Therefore, you could just start business and see if you need to form a company or LLP later.
To protect the domain name, get it registered. You may want to consider registering some of the variables (.com, .co.uk, .net etc. if you think it will be that neccesary). Ozzy could probably help with that. To protect your company name, yes you could form a company or LLP to protect it. If again though you believe that the name requires protecting properly you will need to consider trademarks ('crossguard' on this forum is very helpful with this).
I hope that this helps.
Mike.