If you've come from the HL1 mapping universe, you'll either get HL2 mapping, or you won't. I've seen many a great HL mapper who just cannot adapt to HL2 mapping.
As for tips, you'll want to try and realise what Source's limits are, just as you would for HL1. I started in HL2 mapping by making a really small map (an alleyway) and detailing the heck out of it. Because it was small, I was able to really push the detail aspect without it taking a huge amount of time.
Have a good look through the prop models avaliable to you. Prop models are a crucial part of Source mapping. Basic knowledge of what props are avaliable to you will help you, I assure you: you'll know what you can do, and you'll know that there's a prop there to help you.
As alex said, use inspiration. Try experimenting, but don't release anything. Your first few maps (unless you're damn talented) will be pretty junky/buggy, but they serve as a starting point nonetheless. Read up on Source mapping (check our Links page) for some ideas of what the Source engine can do (bumpmapping, HDR, etc).