First you need to find what mapeditior is best for you...

If your a Windows user you have these as your best options.
Worldcraft (probibly the most used. Most of the tutorials linked below are for this editor)
Quark
Quest
Stoneless (similar to workdcraft... but not as developed or polished)

For Mac you have pretty much one choice only...
Quiver


Considering I, (TheDumbAss) the author of this site, am not an accomplished mapper I beleave I should leave this to the 'professionals' Below are links to several sites with information about editing your own maps. Some of these links may go to sites related to Quake2 or Halflife map making. Most of the info is interchangeable and would be helpful to the average Quake1 mapper. I know I've learned some new techniques from reading through some of these.

When you are first starting out I'd recommend against worring about the more advanced techniques... they'll just confuse most people. Once you've gotten your feet wet and have tried to compile a few maps THEN you should check them out.

"The Forge"
[ The official site for worldcraft 1.6 It details many things involved in making your own maps for quake1 and/or quake2 using worldcraft. The site includes some sample maps to show how some of the (otherwise abstract) ideas like a switch door or "path_corners" work. ]

Quake Editing Tips
[ An older and somewhat short question and answer session with American McGee, Tim Willis, Jim Molinets and a few others. Some of this info is now well known to mappers but there may be a thing or two newer mappers don't know. ]

Rust - Level Design Tutorials
[ A collection of editing tips and tutorials for games based on a various engines. This link goes to their Quake2 secton. Many of these tips would be helpful for a Quake1 map maker to use. You may want to check the Halflife and Generic tutorials.]

JCH Headquarters Advanced Half-life editing FAQ
[ Aimed at halflife there are many things that are not useful to the Quake1 mapper. Some of the things are still very useful to the Quake1 mapper. Its easy to fallow "FAQ" setup will answer many of the questions mappers have. ]

BMP2MAP
[ Not really tips or tutorials... This is a link to a utility that can make simple maps out of nothing more than a BMP of the floorplan. It would take a bit of work to get it to create maps that look exactly how you want. BUT it is useful if you have a large single story map and don't want to build it from scratch in your map editor. ]