Everyone has to start somewhere and there's certainly no shame in being a beginner. Poker is a complicated game and newcomers are bound to make a lot of mistakes at the beginning of their journey. Some of those mistakes are fairly universal.
Without the proper bankroll management, poker is just gambling. Just like you wouldn't bet everything you have on a single business investment, you shouldn't play with the entirety of your poker bankroll. This is a fairly basic poker principle that even most of the beginners are aware of, but just because you know that bankroll management is 'a thing' it doesn't mean you're out of the woods yet.
Even in 2017, it's still incredibly common to see the same bankroll recommendations that were around in 2006 where some basic common sense basically guaranteed a 10bb/100 win rate. 20 or 30 buy-ins per limit in cash games is not a suitable recommendation for a beginner as it's based on the old risk of ruin calculations made in the world where said 10bb/100 win rates were much more common.
Besides, no amount of buy-ins will prevent a beginner from going broke if his win rate is negative (which is most likely the case given that he's - well - a beginner) but airing on the side of caution can certainly soften the blow. No matter how well off financially you are outside of poker, and no matter how much you're willing to deposit in your poker account, start your poker journey at the lowest limit possible and make your way from there.
Poker is a complicated game, and if you're convinced that the only thing you need to do to get good at it is to play it you might want to think again. It's true that depending on your personality you might be one of those lucky people who learn best 'on the job', but if your fundamentals are lacking you won't even be able to identify your own mistakes. If you're just starting out focus on honing your basic poker skills. Learn about starting hand selection, table dynamics, basic post flop lines, player profiling, odds and outs.
Instead of getting the feel for the games by playing the game, get it for free by watching coaching videos. It doesn't mean that you shouldn't play poker at all, it's just that for someone who's just starting out proper education to playtime ratio will be somewhere in the neighborhood of 3 or 4:1 and yet almost no one ever followed that advice. Do yourself a favor and be one of the few ones who do.
Too Many Tables
Once you decide to hit the tables, make sure that you don't overwhelm yourself with the number of them. Most beginners actually start with one table, because that's what seems the most natural for them, but as soon as they learn that it's possible and even desirable to go past that number they promptly decide to double down on the idea.
The truth is that most of us - including the seasoned players - tend to overestimate our ability to multi-table and most of us would benefit from cutting the number of tables that we're currently playing on by one or two. Because of that, instead of doubling down, build your way up gradually.
Fancy Play Syndrome
This is a classic. Beginner poker players are often hellbent on channeling their inner Tom 'durrrr' Dwan and showing the poker world 'how it's done'. Online micro stakes cash games are full of players acting like they're on the next season of the High Stakes Poker show. Don't be one of them. The reality of micro stakes is much more mundane than most players are willing to accept.
Even in 2017, a tight solid game is still what's going to make you the most money. Instead of slowplaying or check/raising every other hand, make sure that your cbets are profitable and that your turn and river ranges are strong. Don't rely on other players to build the pot for you, instead, go for a basic yet powerful bet, bet, bet lines with an obvious value bet worthy hands. Instead of going for giant overbet bluffs, go for small, precise thin river bets.
Falling Victim of Their Own Ego
This is somewhat tied to the last point but it concerns more of the mental side of things. The reason why many players tend to favor fancy play over strong fundamentals is their ego. Our need to feel superior isn't exactly compatible with the harsh and volatile reality of poker. Variance doesn't care about how brilliant your slow play was nor does it concern itself with your self-image. It just is and you should be the one adjusting to it because the other way around is simply impossible. The truth is that you don't have to be the next Dalai Lama to beat NL10.
If you're just starting out learning proper strategy should be your bread and butter. However, you should try to beat the basic mental issues that poker reveals to the punch. Most of the beginner players learn about the importance of the proper poker mindset once they encounter severe issues. To remedy that basic mistake, include some books or articles about the mental game in your training regiment.