FAir -> Down Smash. The only Aerial you need to approach with when your opponent is on the ground is a Forward Air. Everything else has too much landing lag or (or in Up Air's case, a small and limited hit box/knock back). You'll have trouble with shorter characters though. Your Forward Air should last the entire attack animation while in the Air, and landing with little to no lag into some ground attack. Down Tilt, Up Tilt, Down Smash all have enough speed to take a Combo forward.
Neutral Air takes a bit more practice. It has a "sweet spot", per se. It requires forward momentum to give it knock-back. If you were to drop straight down on someone, the knock back would not be as good as if you were to full dash, short hop, Neutral Air forward. Just really think of it as kicking a ball.
Back Air has few uses. It has a lot of ending lag, but the start up isn't bad. It requires you to make all of the hits connect to be useful. It could be a decent Edge Guard with enough practice if you can connect it, and drag, in a sense, your opponent downwards with the attack. I would never recommend approaching with it though, and only use it on the stage as a last attempt when you're facing the wrong direction or something because the landing lag does not win over the stun that it gives an opponent. Even if you hit them, when you land, they'll most likely be on the ground to beat you one.
Down Air is very situational. It has good priority, and knock-back, but it does have a lot more lag than Forward Air when it lands. It gives a shockwave on the ground, which can help against someone's offense, but it can easily be shielded as well. It ought to be used when you're approaching from above as opposed to the sides. In most cases, Forward Air would be more advantageous. It has multiple hits, and stabs and depletes shields much more efficiently, and you can string it into the monster Down Smash.
) and i'm having a hard time adapting to brawl. I want to be able to beat lvl 9 cpus without breaking a sweat.