"If it was easy everyone would do it" -
I don't mean this to come off as condescending, but there are no easy answers. I have been doing this for several years, and I am still learning, adapting, and refining my processes. I am sure several of the vets here would echo that sentiment. But there are tools, sites like this one, and others, that can help you learn and grow. But like most things, the best teacher is practice.
Even without insider info, there are ways to make educated guesses as to what is retiring - there are some predictable trends, and not so predictable trends - For example, we know what themes LEGO is still using in 2019, and we can see that "LEGO Batman Movie", "Cars", "NEXO Knights" and "Elves" are not among them - so we know those sets are retiring.
From themes that are continuing, such as DC Super Heroes, LEGO typically only has one BIG SET (140 ish price point) for each super hero line, currently Flying Fox for DC And the Sanctum for Marvel. Flying Fox has been out for a year and a half (Release dates available on Brickset.com), so it's an educated guess to assume that set is on its way out, even without having any other info.
Some other info we do have on this set however is that it's been cleared out from most major retailers at steep discounts. That also can usually be taken as a sign of something retiring.
Every set retires eventually. Some sets bring better returns than others at a different timeline and for different storage spaces. All those nuances are things you'll have to figure out yourself. From my example above, I think most serious LEGO investors are shying away from most LEGO Batman Movie and Cars sets, even at steep discounts. Elves seem to be a mixed bag, and I find NEXO Knights interesting. After the relative flop that I consider most of CHIMA to be, I avoided a lot of NEXO - And some of those sets have actually appreciated quite well - and I just haven't put much into the theme.
But as someone else said, you can't buy it all. And some decisions will be winners, some will be losers. If you have more winners than losers, you're doing well. Personally, I have more Joker Steamrollers and Gorzan Gorilla Strikers than I care to admit. But I have far more Gorilla Grodd goes Bannanas and Joker Manors to make up for it. And here's the thing, MOST "losers" still sell for close to MSRP, so if you bought at a decent discount, 40% or more - You're still going to make a few bucks or come out even. That's one of the things I like about doing this, since at worst you are likely to break even - It's a pretty decent risk. The sets I have bought over the years that sold for less than MSRP 2-3 after retirement I can count on one hand.
If I were to give advice to someone just getting involved, I would say this: Have a budget and a plan. Don't overspend. Be realistic. And definitely do not buy every set that's retiring or every set that's 50% off. THERE IS ALWAYS SOMETHING TO BUY. There are better/more efficient ways to go about it all, but as we all keep echoing - things are changing constantly. The Amazon gating thing is kind of a big deal. I don't even know if I *could* start doing this today with a goal of making money. Once you're already entrenched and have a system/rotation, it's a different game.
Even as someone who has done this a while, and who (I consider) to have decent success, I would be lying if I said I totally understood it. I really thought 31038 Changing Seasons was going to be successful, and it has turned out to be pretty much a dud (even though I personally love the set). I would have never guessed that a set like 31032 Red Creatures would have been the smash that it is, but I ended up with a fair amount of them anyway.
You are here, which is a major step. I was doing this for years before I discovered this site, and the information and experience found here have been instrumental in helping me grow. So from here, I'd say do a little research, dip your toes in, and see how it goes.