To get started maybe review some maps you know are good or interesting - while you wait for those made with TB to get up to speed.
http://www.quaddicted.com/reviews/
Is where they are, but there's a lot :)
So I humbly suggest these;
The Living End by Necros (Grim medieval map crawler)
Zerstorer Turtle Map pack (excellent map collection linked to a classic SP mod)
The Castle of KooHoo by Vondur (Legendary)
Concentric Devastation by Lunaran (epic, single level mod)
The above is a decent cross section that anyone new to Quake would want to know about, although maybe a bit on the epic end of the spectrum.
Of course, if you get bored / are very enthusiatic you could try doing some of the multiple map packs / mods as well;
Rubicon2
Insomnia by CZG
Nehahra
Something Wicked this way Comes by Tronyn / PM
Operation Urth Magik
Contract Revoked by Kell
In The Shadows by Sock