how can i make our mirror Ricta to be first choice in rwanda and i can only use it by editing source list but i can't find it when i am installing new machine then choose rwanda i don't see my mirror what can ii do to make our mirror the first choice

Have you read https://wiki.ubuntu.com/Mirrors ? There you find information what needs to be in place to qualify for an official "normal mirror" and a "country mirror" server.

Remark: As far as I can see your server is already registered at https://launchpad.net/ubuntu/+mirror/rw but it currently provides only Ubuntu xenial amd64 via ftp. It seems to be still missing certain features and data to serve as official repository server.

what are the features missing exactly if are the guildlines you can share it with mee
thanks

I have nothing to do with mirror administration, so please do not interpret my comments as official answer.

My "missing features" comment mainly refers to country mirrors. According to https://wiki.ubuntu.com/Mirrors a _country mirror_ has to have http activated. Do you intend to serve as country mirror (rw.archive.ubuntu.com) or do you just want to be listed as local mirror that is offered in the installation and repository set-up dialogues?

In any case, the data for xenial i386 architecture and all data for releases different from xenial are missing on your system.

Maybe you should try the contact information given in https://wiki.ubuntu.com/Mirrors