The list of .APP registrars should supposedly appear at https://www.registry.google/about/register.html at some point.
In the mean time, 2 options:
- either contact your registrar of choice, the one with whom you are already doing business, that you know and appreciate; they should be able to at least help you and let you know if they will either provide this TLD at some point or give you advices on where to go to have it
- consult the ICANN list of registrars at http://www.internic.net/regist.html (a very ugly website, but the authoritative source) or https://www.icann.org/registrar-reports/accreditation-qualified-list.html, as for newer gTLDs such as
.app registrars have a very lightweight procedure at ICANN to become accredited in them; or just start from the Google list of registrars for their other TLDs as there is a good chance they will carry
When a gTLD opens it has to (ICANN requirements) go to specific launch phases, first one for trademarks, called the sunrise (which can be further split based on various criterias) and then one for "claims" where prospective owners will be notified in case they may be infringing some trademarks, in order to give explicit consent before going further. This later "claims" phase can run undefinitively.
During the trademark case, to apply for a domain name, you will need to get an SMD file that you obtain after registering some kind of trademarks at the TMCH office. Some registrars or trademarks agent can provide you this service for a fee.
Also in case of difficulties finding regitrars you first point of contact should be the registry itself. So try to contact them and see how they can help you.