Hm... I just got into the DC Universe myself (although I've been reading a little Vertigo for a while now), and I've been flipping through pretty much all of the OYL titles for the past couple of months, to decide what I want to pick up. So here's my stance on it.
For the mainstream DCU:
BATMAN and SUPERMAN are great places to start. The opening OYL arcs for both are pretty interesting, and they're both getting kickass writing teams in a month or so. They're pretty light on continuity, meaning they're easy to get into, and provide fresh starts on the mythologies. The only downside is, they require an investment of two issues per month (Detective Comics and Batman, Action Comics and Superman).
WONDER WOMAN started a few days ago. It's got a new Wonder Woman in the title role, and it looks like it might be fun, plus it does a pretty good job explaining the background. FLASH is restarting with an old iteration of the character in a week or so, but I have a feeling it should be pretty fresh continuity-wise. I hear Flash is usually one of the most consistent DCU titles around. AQUAMAN has a new character too. I haven't been a big fan so far. It's sort of Conan-ish, and it looks like it will be pretty self-contained to the underwater kingdoms and the like. CATWOMAN has a new lady taking over the role, but it's never been a concept that's really grabbed me. GREEN ARROW is fun, but it seems a little like a watered down Ex Machina. NIGHTWING is okay if you're looking for more of a dark-edge style comic. BLUE BEETLE comes off to me as a weak-*** take on Spider-Man. But the three core books (Bats, Supe, WW) seem like the best for solo heroes.
Now for team books in the DCU. TEEN TITANS seems like it will be fun. I don't know much of the background on the team, but it seems like a fairly enjoyable dramatic superheroes series. OUTSIDERS is uber-cool and focuses more on espionage and underground missions. Sort of the A-Team of team comics. JLA is restarting in a month or so. It's really the A-List of DC team comics, and I figure it should definitely be checked out. JSA is rebooting in a month or so, but I'm not exactly sure exactly what that book's going to be like. LEGION OF SUPER-HEROES is garbage. SHADOWPACT is well-written with some hit-or-miss art. I'd pick it up if you're really interested in a books about magic superheroes. SECRET SIX is excellent. I know you're only planning on picking up 3-5 DCU books a month, but I'd suggest getting that additionally. It's only going to be a 5-6 books miniseries, and it's righteously excellent.
I've never really gotten in to the whole angle of the Green Lanterns, so if you're thinking about picking up GREEN LANTERN, ION, or GREEN LANTERN CORPS, you're gonna have to get help from someone else.
52 is cool as hell, but it's also a bear of an investment. Four issues per month.
I'm guessing you're just asking about the DCU, so I won't mention Vertigo, but there's some good stuff there too.
Normally I'd suggest you pick up both Batman and Superman, but that's a four book investment, and you're looking at 3-5. If I were to make a suggestion to you, I'd tell you to pick up these books:
- Either Action Comics/Superman or Detective Comics/Batman, whichever line interests you more.
- Justice League of America (not yet started)
- Either Outsiders, or Teen Titans (I personally prefer Outsiders)
- Flash or Wonder Woman (again, depending on preference, but I have a feeling the new Flash is going to be badass)
- Secret Six, which is just a mini, and hopefully not too much of an investment.
That should give you a pretty well-rounded view of the DCU as a whole.