Hello man! Welcome to CC! We all hope you will enjoy it here with us
I was in similar position about half a year earlier, I wanted to start playing more seriously and try to climb the limits so here is some advice from me:
1. Stakes
I highly recommend starting at the lowest possible stakes at the site you play. Many new and inexperienced players get fooled into playing higher (like I did) but its just a waste of money. If you think you are better than 2NL, just play it anywas and see for yourself if its really true. In my instance it was not true and I had to learn a lot to beat the lowest stakes and Im still in fight with them
If your skillset is strong enough to play higher, just beat this limit first anyways and move up gradually.
2. Multitabling
You should do what suits you the best and no definite answer here is needed. Try for yourself. If you are a new player you should start with fewer tables, if you are playing regular tables and not zoom, I think 3 or 4 tables are sufficient. If you feel you are not focused enough when you have so many tables, just close some and try to play only the amount you can focus on. I had problems with this, now that I played a few thousands of hands I improved my multitabling to playing 6 tables at a time. Just start off small.
If you play zoom its fine to open 2 or 3 tables and you can fold or play as fast as you want.
The thing with multitabling (at least for me) was when I had only a few tables I was bored and I did a lot of mistakes out of impatience. However, when I played too many I was too distracted as it was all too fast for me. So try out and find what suits you the best
3. When to leave the table
This is also very different from player to player but I would just advise you to leave as soon as you feel tilted or emotionally affected (angry, too hyped up, bored, upset) or affected in any other way (in pain, exhausted, sleepy, ... ). If you are not 100% focused you will not play your A game which is not good. I saw some players use some kind of stop-loss system to leave the table, meaning, for example, lets say your stop loss will be 3BI so you play and when you lose 3 Buy ins for your current limit you quit. I dont use this personally but it might be good for you.
4. General improvement
If you are a fun player then this might not be interesting for you but if you want to play to improve and make some money or move up limits, this will be the most important part of advice. You should always keep in mind you need to improve your game and that means not only playing, but studying, analysing, etc.
There are a lot of materials and interesting threads here on the forum, especially in cash game subforum (here). You can find some cash journals of our members (including mine) and learn something from our mistakes, ask questions or what ever you like.
You can also post your hands in the hand analysis sub-forum. You will get a reply for sure!
There are tons of studying materials on the internet. Go search for some
If you want some suggestions just tell us and we will be happy to give you some hints on where to start finding articles and studying materials.
GL!