Well, here's my experience (might be outdated):
The only way I could ever find anything that isn't already on my radar was AWACS. There should be a request for closest enemy or similar. I believe requesting a picture lists all enemy contacts starting with the closest. AWACS should give them to you in BRAA (Bearing, Range, Alitude, Attitude [pronounced brah]) format.
The su-27 radar display shows friendlies as two lines of the little rectangles and enemies as one. Also FYI, 2 little rectangles is a medium sized contact (fighter), and 4 is a large (bomber).
As for locking onto them, you should probably read the controls. Really don't understand why anyone would jump into a game like this without at least looking at the bindable controls and then wonder how to do something. There should be targeting cursor (or something like that) up, down, etc. and target lock and unlock under sensors. Read the rest of it too, they didn't slap things like airbrakes and external lights on these things just because they were bored.
If you have any other questions about controls I'll probably be able to answer them. I only mapped my CH HOTAS to each plane about a hundred times.
As for reading the controls, trust me, I have. The very first thing I do is go through the controls and training videos. I have pages and pages of checklists for every DCS aircraft, with both the english explanation, and the Russian characters to look for in the cockpit. I like the realism, and part of that for me is knowing every switch, every dial, and approaching every DCS flight like I approach my real life flights.
That being said, the resources available for the Su-27 are not on par with the Mi-8 and Ka-50. I don't know if you've tried the Su-27 campaign, but that's the extent of my Su-27 experience, so I'll use that as a reference. There is a mission where you have to intercept a pair of drones. The waypoints are the same rectangle as every other mission, and the entire time you're up, you're getting a constant stream of enemy contacts, but you're not authorized to engage those in the ROE, you're only hunting two predators. I ended up finding them on my F10 map, and thus finding them on my radar as well, but in BVR mode, while I had a targeting cursor, I had my radar on, I had the predator right on my nose (Icons were turned on), and I had the targeting gate over the predator, the lines that identify an enemy were not on the hud, and my lock target button did nothing. I kept closing, no dice, finally I got close enough to go boresight, got the lock with that, and shot. I also verified I had radar guided missiles selected and not heaters.
The next mission, I'm supposed to escort two flights of Su-25s on a strike run, but the waypoints are the exact same rectangle, which did not include a path to escort the Su-25s. I flew the rectangle, and only found the Su-25s with the F10 map again. I followed them, and without any warning (or radar contact) I see F-16 icons ahead. Again, can't lock them BVR, so I close, boresight and a kill, but now I've lost sight of the Su-25s, and I've got a bunch of icons on my radar, but I cannot fight an explanation of how to read the radar screen icons, so I don't know what I'm actually looking at.
I'm certain there are bits of information I'm missing, and I'm relying on the F10 map far too much. I've repeated the training missions ad nauseum, but I seem to still be missing some fundamentals, and while I'm constantly reading there forums, I seem to be starting WAY behind the power curve.
