You can write either for all male, all female, or mixed. Most common is major key stuff, minor is tricky to make sound 'bbshop'. Use nice round simple chords, lots of 1-3-5-8s. Solos, bells and doo-wahs are restricted to a few passing tones or 'pickups', mostly you should have locked chords all four voices working 90-95% of the time.

The lead voice takes the melody most of the time but occasionally passes it off to the other voices, and is the second highest from the top. The top harmony line 'tenor' tends to take a lot of 7ths and 3rds and 'rubs' (right next to lead note).
The bottom voice tends to take the tonic or 'key' note of the chord as much as possible, makes for a 'stronger' chord. also okay to use the fifth. The fourth voice is baritone, it is the garbage or filler line which completes the chords you need to make.

A 'barbershop 7th' is a flatted 7th, not the other one. 9th chords are also standard, often with the root missing.

When you write in bbshop style, it is common to use the two staves treble and alto, put two voices on each staff, one with stems up and one with stems down. Tenor up, lead down. Lower staff bari up, bass down.

If you write for all male, the top staff is actually sung an octave lower than written. This allows you to cram a bunch of notes close together audibly but be able to see them on the paper as if they were all spread out. For chicks, it is the lower staff which is sung an octave (12 semitones) higher.

Google around to find the SPEBSQSA and Harmonet websites and drill down for more info on arranging.