No worries! MCU is totally sufficient.
The cinematic universe has a wiki over
here with decent articles on characters and history.
I'm not sure if it's canon in MCU, but traditionally Clint is deaf and learned archery in a circus (can you tell his backstory was written over forty years ago? P: ) he and his brother joined after running from a drunk dad.
This has a really well written Clint, a blend of the movies and modern comics--a little more of a shmuck than the movies show, but nails his humanity perfectly.
We Were Emergencies by gyzym on AO3 is an *amazingly* well written Clintasha fic that develops his movie character--it's also incredibly filthy, thus why I didn't link it. The characterization is absolutely flawless, but it is a long read. And like, so porny. But it delves into how Clint's possession would be psychologically scarring and, gah, it's just so good.
Sif is more steeped in myth, so you have all that to draw from. I really like how she's written
here. This author does such a good job of writing the ladies of MCU, haha! Her piece on Maria Hill is the single greatest interpretation of her character ever. All her pieces stick strictly to MCU canon.
Sif's wikipedia and MCU wiki pages have really good information on her character, and I nodded to her and Heimdall being siblings in my post. You are free to pick and choose whatever canon you fancy, it's cool. :)
Mostly, I just read tons of fanfiction, some meta on tumblr and rewatched the films. From there, it's just kind of going crazy with it. It's Marvel, it's all weird and kind of silly, whatevsies.