This may seem like the Lazy Man's answer, but check out the War Academy articles, read the strategy guides, especially some of the starter ones. Look for threads that ask questions on topics you're unsure about yourself. If you're not yet winning Noble, then there's still several basics of the game that you have yet to fully comprehend. That's not an insult by any stretch of the imagination, it's a nod to the sheer complexity and depth of field for every aspect of Civ4.
I'd say by the time you can consistently win Noble with random leaders, you have a solid comprehension of most gameplay elements in Civ4.
Once you can consistently win on Monarch with random leaders, you have a solid comprehension of gameplay strategies.
Once you can consistently win on Emperor with random leaders, you have a solid comprehension of military strategy and critical timing.
Once you consistently win on Immortal with random leaders, you have a mastered all of the above and have them down to a science.
Once you consistently win on Deity with random leaders, you have it down to an art.
I myself am only "doing okay" at Emperor level, meaning I'm still trying to get a firm grasp of military strategy and critical timing, even though I've got a pretty solid grasp of gameplay strategies and elements.
I would recommend that, for now, you simply focus on learning the ins, outs, and hard numbers behind each gameplay element (such as the difference between commerce and gold, city maintenance vs. civic maintenance, mapping out your wonder attempts ahead of time, when to whip, how to choose (or not choose) the best civic for your situation, etc.).