This is of course a very in depth topic that in my opinion I know very little about, so I'll try to summarize as best as I can

. Most of these questions can be answered with "it depends" based upon who you are as an investor.
In my view, it is best to have a blend of safe long term investments, as well as some short and medium term if you have the time and money to spare. For the long term, the balanced approach would be to have the bulk of your investments in broad, diversified ETFs or index funds. For example, SPY covers the entire S&P 500, QQQ is the largest 100 non-financial companies on the NASDAQ, etc. And of course you can also pick individual companies that you believe in on top of that. From there, you can then have your play money to throw around as you please in short or medium term plays. Here you'll win some, and you'll lose some, so it's important that this is not money that you can't afford to lose.
With that said, what are the short term plays? Well, this also depends on your personal risk tolerance and timeframe you have in mind. If it's a few days to a week or so, you're going to have to find something that's very volatile or maybe heard some good rumors ("buy the rumor, sell the news"). Take meme stocks such as GME and AMC for example. Highly volatile and obscenely overpriced, these are things that you would buy into ONLY if it wouldn't hurt to lose that money, because these can crash to normal price levels and never come back without warning. Personally I would never touch those right now. Cryptocurrencies are imo another option for this, with double digit percentage swings within minutes sometimes (although many consider this a long term investment). Things like these you would need to be actively watching the ticker, or have stop losses set. It's essentially gambling, and so I tend to almost always avoid it. I made like $60-80 on SOS in a week or two, but also broke even or lost a few dollars in DGLY, GME, and AMC.
I mostly invest for the medium and long term, where I put money into companies that I think will do well or industries that I think will do well. As perhaps the biggest example, electric vehicles were huge this past year or two. Look at the 5 year charts for TSLA, NIO, and PLUG, they exploded because of a renewed focus on the environment and a recent push to shift from gasoline powered to electric vehicles globally (Biden policies, China's EV mandates, etc). Predicting that overall trend and having a little patience could've made you some serious money in this case. I only had one share of Tesla at the start of the pandemic, I kick myself for not buying much more

. A good starting point would be to just think of large companies that you deal with, have seen, or have maybe heard of often, and then do a little research. For example, Google, Apple, McDonalds, BlackRock, you get the idea.
For what to avoid, the two biggest things I would say are to avoid FOMO and don't get too greedy, I'll give you personal examples of both

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All of last year, Cathie Wood's ARK funds were EXPLODING, ARKG had over 200% growth that year. After a takeoff like that, I finally caved and said to myself "I need to get in on that!" Despite the fact that usually when shares jump in price like that, they usually drop off afterwards, I bought in January at the all time high of I think $110/share. We're now sitting at $87, I probably won't be getting that money back.
Also around this time you had meme stocks take off for the first time. I was watching it all happen from the sidelines, telling my friend "maybe we should buy into GME" when it was at around $60/share. The next day it hit $100 and I thought "well I could risk $100" and bought one share. In addition to this, I also had bought 30 shares of BlackBerry a while prior to this when hearing about their Amazon deal and bought in as a long term bet. They both went parabolic with GME over $300 and BB around $25/share. I knew I should've sold them, but didn't because I got greedy and was hoping for more gains. I would've made around $1,000 if I had just sold then, rather than nothing now. So as you can see, I'm an idiot so take from the next paragraph what you will
As for interesting stocks:
I currently have this chilling in my account at the moment. I also have some shares of the BETZ ETF, which gives exposure to sports and online betting companies, as I see this only growing in the United States in the coming year or so. Everything else I own, I've just been holding for a while. Moody's has been absolutely phenomenal for me since the start of the year through the volatility of pretty much everything else.
@Bjerknes and
@lgorTudor probably know quite a bit more than me, but that I think sums it up.