I keep asking questions, sorry!
What are advancement points and how do you use them?

on uses of ap, see this thread

AP are gained by basically doing playing-the-game type things- progressing the story, doing quests, doing missions, battling, breeding pets, nurturing and such (how many AP can vary by situation, eg, from battle how many you get depends on your pets' levels vs enemy levels, the weaker they are compared to you, the less you get, it takes a certain number of nurtures to get an AP, sorts of things)

Please keep asking questions anytime you have need. The community is very helpful, and we all love "'paying it forward" as most of us were helped previously :)

For AP, Scath mentioned these already but I just wanted to add from my experience when I was a new player that I found them most helpful for wardrobe items on my base avatar (hair styles, eyes, etc..) and I used a lot of mine to buy more design slots, for the generator. I find it super fun to design pets that I may or may not use. And we usually have contests during festivals to design that particular festival's theme pets. I can create the early and have it ready to post when it is festival time. And you can set them for public or private if you want others to see or not.

One of my favorite uses for ap points is unlocking more party slots. I am a heavy competitor in battle boards during fests and having a team for each species helps tremendously. If you have enough slots for each team's set up it saves you time when it comes to switching between them during the competition itself opposed to finding them in your stables and adding them to the party. Saved time means more points on the boards. If you plan on every doing boards, I definitely recommend that you invest in those slots.