The first reply has said pretty much everything that you shouldn't be taught.
Firstly... dair is not a reliable edgeguard move. Do not try doing this unless you are willing to go for some extremely hard and precise reads, and also do not mind the possibility that you will not get back onto the stage against good opponents.
Secondly... usmash is great, but it is more situational than is being suggested here. It is a move designed to punish people, not to be the bedrock of a competent strategy. If you want to get a feel for this move quickly, you'll need to play against good Mario and Peach players. (Don't fight Jiggs because that's too easy.) Why? Because frankly, those two are some of the few fighters in the game who will actually bother to close distance with you. In most matches, because of the nature of G&W, you are going to have to do this yourself, and you can't expect to approach Duck Hunt throwing out usmash. Fundamentals like spacing and approaching with shield on the ground are what will win matches for you, not smashing.
Thirdly (THIS IS IMPORTANT)... do NOT use dtilt! This move is extremely punishable because of its high endlag, and you are better off doing something else most of the time. It is only acceptable as an unexpected mixup in situations where you must quickly punish someone out of ftilt range without risking any movement. These situations are far and few between for a character that needs to be aggressive, and recognizing them is not something you should be doing in your formative stages of G&W play.
So what's important for you to learn at this stage, then? In order of importance:
1. Fair
2. Dash attack
3. Ftilt
4. Grab
5. Nair
6. Bair
The first three are your bread and butter moves. They may look easy to block, and they are... but they are actually the same tools of a competent approach. Your first order of business is to learn why.
Grab is the next important thing to learn. It is hard to do, but you have one of the most rewarding dthrows in the game if you can manage it. This is because you can almost always follow it up with a well aimed nair. However, you also have to learn the percentage ranges at which you can do this to certain characters, what your other guaranteed options are, and the feasibility of going for an untraditional followup.
The last two are the cornerstone of your aerial punish game (dair is in there too, but you need a strong understanding of opponent psychology, don't bother with it at first). Nair is for punishing aerialized opponents (most of your moveset is, but do this first so your opponent airdodges predictively) and bair is used to get easy (mind this word!) ground punishes. They have other uses, but focus on these at first.