When you get Half-Life, you might want to get the original rather than Half-Life: Source. The original game uses the GoldSrc engine, while the Source version uses the Half-Life 2 Source engine. However, the graphical quality in the game is still the same as the original GoldSrc version, with only the addition of ragdoll corpses, a few new skyboxes, and extra glitches.
There's meant to be a fan mod coming out, called Black Mesa, which will be Half-Life 1 completely recreated in the Half-Life 2 engine. Problem is, it's been under development for years, and was actually supposed to be released 3 years ago. Its vaporware status has gone from "slow updates" to "running joke" to "what the hell, are the developers dead or something?"
As for Blue Shift and Opposing Force, they were made by Gearbox rather than Valve, so their canonicity is a bit iffy. There's nothing in them that directly contradicts canon, but nor are there any indications in the later games that they're canon. They are, however, quite good. They definitely feel like expansions to the Half-Life 1 experience. You can decide whether you want to get them after you finish Half-Life 1. Personally I would rate Opposing Force a 9/10, and Blue Shift a 7/10. For comparison, Half-Life 1 gets a 10/10 for me.
Then comes Half-Life 2, definitely play that. And then Episodes 1 and 2. Don't miss those, especially Episode 2. And then when Episode 3 is released, play that. Of course, it's a tossup which will come first, Episode 3's release or the heat death of the universe, but we Valve fanboys like to keep our hopes up.
Finally, as Pez mentioned, playing the first Half-Life games isn't necessary for understanding the Half-Life 2 plot. In fact, a lot of people had trouble seeing the connection between Half-Life 1 and 2 when they played it the first time. Which is actually a deliberate gameplay decision on Valve's part, but... well... now I'm blabbering.
EDIT: Oh, I've also got 2 Valve coupons for 25% off, you're welcome to them if you want them.