Which are most common mistakes when you play poker live first time? Any tips for the first newbies? When this "situation" ends I must try live poker right away!

lol,
you have to remember you're not playing online and hidden behind your computer, so if you start talking shit and bad mouth people, it may back fire on you, and if you start with those BS like NH, etc...
just don't do it.
Keep your poker face, keep your focus and don't do small talk, and do not show them your nervousness, do not show your emmotions, your weaknes....
Take care of your breathing, finger tapping, all of those tell tell signs...

Most common mistakes are probably things like playing out of turn and betting/raising incorrectly.

This is because online, these things are automated for you so you cannot play out of turn or raise too small (i.e. less than double the previous bet), whereas live it is very easy to make these mistakes if you aren't paying close attention to the action/table.

With regards to betting/raising/calling the best thing you can do is to verbally state your action before putting any chips over the line.

Verbal declarations overrule chips put forward.

For example, if you intend to raise but put forward only enough chips to call, if you did this action silently- it will stand as a call.

However, if you declare "raise" first, and then put forward only enough chips to call- your verbal declaration will stand and your bet will be amended.

This also works the other way round- if you accidentally put out three chips totalling 1,500 (without saying anything) and you intended to just call a 500 bet, this will stand as a raise. If you had stated "call" before putting the chips out, your declaration will stand and you will get the 1,000 chips back.

That might sound stupid and you think you'll never accidentally put out 1,500 when you meant to put out 500, but you would be surprised how easy it is to make such a mistake. You can mishear the player or glance at their bet and misread the chips..

On this note, if you put out a single chip of a higher denomination- it's a call, whereas multiple chips count as a raise.

So, if the bet is 1,000 and you silently put forward a single 5,000 chip- that is a call and you will be given 4,000 change from the pot. If you silently moved five 1,000 chips forward- that will be a raise to 5k.

To avoid misunderstandings or 'misclicks', it is prudent to verbally declare your intended action before moving chips in.

Don't splash the pot.

Don't touch other people's chips without permission (even if you're trying to help) including their bets, always ask first if you are trying to helpfully break a chip for change or the suchlike.

Don't expose your bottom or throw your shoe at the dealer.

Don't get drunk or be obnoxious.

Generally just try to be polite, respectful and patient.

That's about all I can think of for now.

Damn I miss playing live!

Thanks for advice. I think my biggest problem will be the bet sizes but I guess you have to start somewhere.

Absolutely, everyone has to start somewhere!

You'll quickly get the hang of it, don't worry..

Oh also, don't string bet!

It seems most of the typical mistakes have already been covered. As a tip, I would say, believe it or not, during your first few sessions, if you feel you are at a friendly table, or at least mostly of those seated close to you, you may try making a point to bring up in conversation the fact that you have only played online, and are not as familiar with some of the live etiquette. You may be surprised at how many people are willing to help you out here and there. Obviously, not helping you play your hand or anything, but just things that ultimately helps the flow of the game, which in turn helps you and them.