I'm only 6k KGS, so I may not know as much as other people around here, but this is what I think:
First of all, you shouldn't think things like "I should attack his weakest group". Unless you're really behind in territory and have to try something, that is. Why not? Because the (in your eyes) weakest group, is not necessarily weak enough to warrant an attack. Instead you should think "I should attack weak groups". Notice the difference? The former means there always is a group to attack, because there always is a "weakest" group. the latter means however, you should only attack, when there really is a group weak enough to do so and you get profit in some kind by doing so. You can still attack a not-so-weak group, if you get something by attacking it (territory, strength, etc.), of course. Now, let's take a look at his group. it might look like there's no eyespace yet, but that doesn't mean it's weak. Even if you get a good attack from the top, black only has to play N2 for example, and is more or less immediately alive. And you don't get anything great by attacking, aside from some 3rd line territory on the right. If you attack from the bottom-left (let's say after you connected O2), he can make easy life on the right or even somewhere in the middle. The main problem is, you can't attack this group in sente. it can make life on the bottom, it can make life on the right, it can make life inside, it even has enough space to run, if somehow all else fails. Even if you were attacking from the top AND the bottom without him answering, the group wouldn't be in big trouble, because it has too many options to live.
This brings us back to "I should attack weak groups". The black group on the lower right is not weak. Plus, you can't get anything by attacking it (because it's not sente), so you shouldn't attack it. Play something on the left side, or take the lower left corner.
Deciding what to attack and when to attack is not something you can understand from one minute to another. As I said, I'm only 6k, so maybe I'm not even right with what I'm saying (I'm pretty sure I am, though
) and I have to learn a lot about attacking myself, but what I'm trying to say is: don't always look around the board and try to find something to attack. Instead, play big moves, wait until there is an opportunity, then attack and get something out of it.
Besides, whenever stronger players review my games the mistakes they think are important are ones I didn't even see!
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