If you're asking how to play the note Bb, you play it by pushing the first valve.
If you're asking about what pitch it is, Bb on a typical Trumpet is the same as Ab on a piano.
On a standard Bb trumpet, the fingering for A is 12.
A Bb trumpet is in the key of Bb, a C trumpet is in the key of C. When you play a Bb on a Bb trumpet, it sounds a C (concert pitch). When you play a Bb on a C trumpet, it sounds a Bb. C trumpets are in the key of C or concert pitch
They are in different keys, a Bb trumpet is in the key of Bb while a Flute is in the key of C. so if you wanted them to sound the same have the trumpet play a Bb and the flute a C
Bb is played with first valve if you are using a standard Bb Trumpet.
A trumpet is pitched in Bb. A flute is pitched in C. If a flute is playing a C, and a Trumpet is playing a C as well, the flute is playing a concert C, while the trumpet is playing a concert Bb. They are the same note on paper, but different concert pitches.
The 1st note of a Bb scale, then the 5th. Both are open.
The trumpet can come in many keys, but Bb is the most common.
The lowest written note in the range of a Bb trumpet is a low Gb. The frequency of this pitch is 184.997Hz.Of course, the true "lowest" note is limited only be the skill of the player.
If, by "bb" you mean b-flat, that is a trumpet that plays in the key of B-flat, as opposed to a C trumpet or E-flat trumpet.
There is a link below to a sheet of music with all the notes in the written range of a Bb trumpet. It includes note names and fingerings. Note for non-Bb trumpets: The fingerings for the written notes will stay the same, however the pitches (and names of the notes) will be different.
The most common type of trumpet is a Bb trumpet.