This is what you should do if you want to join a choir in Finland:
1. Find a suitable choir close to you.
2. Google the contact information of puheenjohtaja (chairperson of the singer's association) or kuoronjohtaja (choir conductor), which is also sometimes called taiteellinen johtaja (artistic director).
3. Call or email them and write that you'd like to sing in the choir. You can also ask for a recommendation for another choir.
4. Be prepared for an audition and go to the choir practice, if there is space for another basso, tenori, altto or sopraano.
Here's a model email that you can use: