Just from what I’m reading here, it sounds like you’re feeling a little overwhelmed. That’s normal; so, don’t worry too much. There’s a lot to learn, but you should just start by doing stuff in the game and then asking questions when you run into things you don’t know how to handle or do.
The addons suggested are suggestions, and as one poster mentioned, deal with them when you wish you could do something in the game but can’t find a way to do it or see someone else doing something that you’d like to be doing.
Your initial backpack will remain the same size (until you get double authentication, I think–then you get an extra row. Check your account settings to set that up). You can’t get rid of the initial backpack or use a bag to replace it. It is what it is–hardcoded into the game.
Playing through the initial quests will get you an introduction to the main city along with its important locations. My advice is to click on all the NPC’s just to see what they have to offer. You don’t have to buy anything, though once you find the bank, open up bank slots and put bags in them as you can afford them and need them. Also, there is a reagent bank which costs 100 gp to open up. If you’re doing a lot of crafting or gathering, this is where your reagents will go. When in doubt, you can try to throw something in there to see if it will go. If it does, it’s probably related to a profession.
The game doesn’t do a good job of teaching you about macros. Don’t worry about this too much. Until you get to 70, you probably won’t need them, but if you find yourself having a hard time, just ask for help in the Macros and UI forums. Usually someone can give advice there. I use them rarely.
You said you wanted help with your UI. Check the aforementioned Macro and UI forums. The UI is…a LOT, and half the things you can move around on there you probably won’t know what they are. BUT…move them around. Play around with them. Eventually you’ll figure out what those nebulous boxes are for and find a good spot for them.
Watching videos and the like is fine. If there’s something you see that you like in them (such as a specific set-up etc.) feel free to ask the video’s creator or ask around here. Make sure to let them know you’re new to the game. This will help them to (hopefully) better explain things so you can understand them. But again, if you ever have a question, just ask. MOST people want to help and are nice, and the ones that are jerkfaces you can just /ignore and move on with your gaming life.
As others have said, welcome to the game. It’s a journey, not a destination. Take your time. Enjoy the many deaths that you are likely to experience. Try to learn something new each time you play, and most of all–have fun.
