I'm fairly new to modding too, and it seems like a good way to get started is to just go and tinker with something, and see what it does. Give chickens a poisonous dust cloud, or add in some adventurer mode reactions, or change the stats on an item, or something.
Seconded.
I learned a lot by just trying to give a creature a new body part, that also required new tissues.
Start by doing something simple like adding or removing body parts (not requiring new tissues, I'd recommend), or by adjusting other little things here or there in the creature files. That's where a lot of the immediately interesting and visible things will be. Once I moved on from being able to muck around with body parts, body plans and tissue definitions, I moved on to giving creatures special attacks, like venom. All of this was in .31.25, so I don't know a thing about interactions.
Another thing I'd recommend is doing all of this with the object testing arena. You can spawn your modified critters and inspect them, or watch them kill each other. You can then back out of the arena, modify the RAWs some more, go back into the arena and try again, without having to terminate the program.
A lot of stuff you'll have to figure out on your own, or by asking here. There aren't any particularly comprehensive guides that I could find, although the DF Wiki has a little on it.