Oh holy moly, I don’t know that anyone has a definitive answer to this question? I can give you my recommendation from personal experience though! 🙂
I will say that your best bet for understanding the pre-LOTR material is to start with the regular ol’ Silmarillion.
If you get stuck on the impersonal tone of the Silm (if you remember the appendices in the back of LotR… it’s a little like that), you might have better luck moving to Children of Hurin, or the Lay of Leithian– they both have a more familiar story telling format; more like a novel and less like the King James Bible or Beowulf.
If you LIKE the Silmarillion and the history intrigues you– say, you want more details, more in-depth characterizations, or are fascinated by a character who is only briefly mentioned, or want to know about all the characters who WEREN’T mentioned— then check out the rest of the volumes of H.O.M.E. (those are all the volumes like Morgoth’s Ring, Book of Lost Tales, Unfinished Tales, etc.).
Take it at your own pace– don’t feel forced to read every word of every volume that Chris Tolkien’s published; us weird people online are outliers in the Tolkien fandom, and should not have been counted. XD
Also if you’re like me, you might feel better skipping through the Silm the first couple of times to get to the bits that interest you, and gradually go back and fill in the rest of the story when/if you feel like it.
Try checking out the fandom side of things if you have trouble remembering the characters or events! Look at fanart, read some fic, check out people’s illustrated Finwëan Family Trees… there’s all kinds of weird nerdy shit out there that can help clarify who is who and what’s what. Don’t feel bad if you have to open a cheatsheet to keep track of all similar-sounding elves, each of whom have at least three different names.
Another suggestion I give to people who find themselves sliding off the impenetrable wall of lore that the Silm presents is to go download the audiobook! 😀 A favorite British actor of mine, Martin Shaw, reads it, and he has a lovely voice, great intonation for the character voices, and he reads very slowly and clearly in such a way that a lot of that very dense information becomes suddenly parsable. (Also it is *cough* available *cough* for *cough* free *cough* if you know where to look.)
Godspeed, and good luck!
~Wes