http://tess2.uspto.gov/bin/gate.exe?f=login&p_lang=english&p_d=trmk That is going to be the best for looking up Trademarks. It is the United Sates Patent & Trademark Office.
The USPTO site only shows Federally Registered Trademarks - and isn't going to show phonetic, similar spellings, or foreign spellings unless you specifically search for them. There are also common law trademarks which have no registration and foreign registrations which are honored in the US via the Madrid Protocol. A good trademark search will run several hundred dollars and would be virtually impossible to do yourself. To begin with, search the USPTO and the various search engines. If you find someone else is using the term in the same classification as you intend to use it, they have already established first usage rights. If you don't find any prior usage, you should then considering paying for a TM search. Realize that there is no way to be 100% sure another existing TM holder won't consider your usage infringement - you can only take steps to try and limit future problems. When you file for a Federal Registration, the examiner only checks Federal Registrations, and if it is accepted for publication (this can take about a year) there is then a 5 year period where anyone can contest your filing - meaning it takes about 6 years to have your rights to the term "cast in stone" assuming no one contested your filing. Trademarks often cross over into a grey area, and you should spend some time to research what can be considered TM infringement.