Probably the best level design resource for the Source engine is Interlopers.net. Their tutorials section is quite comprehensive and helpful, and their forum community will answer any additional questions you might have. They have a few beginner's tutorials that will walk you through your first room, basic lighting, texturing, etc.
I would reccomend you as well: Since you?re a modeler and knows how the 3d world works, you should start playing around with Hammer to familiarize with its commands and using the Wiki help that is very comprehensive and helpful as well (just pressing F1 in hammer it will spawn). Also checking the game maps is very usefull to learn how the things works, and hammer includes some decompiled examples for that.
In terms of practising, I'd recommend first making something you are familiar with: it could be a church, it could be a famous place, it could be your house. This is only really useful when learning the very basics.
Next you need to find a game mode you enjoy and make a map for it. This could be a single-player level, a Portal level, HL2DM, TF2 or even a custom mod. Just try not to choose anything with very complex rules. I would recommend making a SourceForts map over a Dystopia or Eternal Silence map. Even TF2 maps are deceptively complex for a beginner unless that beginner really understands the classes well.
If you like Portal, it's a very good place to start, because you don't need to concentrate hard on aesthetic details to make a good, playable level.
After you have made your first playable map, join a mod team or make your own mini-series of maps to gain more regular experience. But take little steps and be prepared to listen to feedback and ignore dismissive comments when you start out. Part of becoming a good mapper is resilience and strength of character.