My best tip for first time live poker players...BREATHE. It can be quite intimidating and overwhelming to sit down at a live table for the first time. I've done it hundreds of times and sometimes I still get SUPER DUPER nervous when I first sit down. I'm talking about the type of nervousness that has extreme physical symptoms like sweating and shaking. I always relax after a few minutes but it can be crazy at first.
Another thing to remember is that live poker is a completely different game than online, ESPECIALLY when you're playing against seasoned players. They will pick up on tells, keep your poker face lol. Also, you can pick up tells on them!!!
Be patient. The hands don't come as fast and furious as they do online. This doesn't bother me, especially at tables with good banter. You don't even have to participate, you can just sit back and be entertained by all the characters haha. Some people may have a different experience though, so remember the hands will be slower.
Be aware of your surroundings and what's going on. Don't be that person who constantly has to be reminded its' their turn. You may need to help push a pot to a winner or move the dealer button.
STRING BETS!!! Once your chips move across the line or even just forward in some circumstances, you have bet. You cannot add or subtract from that amount. Sometimes however, say someone bets $10 and all you have is $100 chip, you can silently place one chip into the pot area and it is considered a call. RAISES should be announced before acting. Be careful with this and always confirm the house rules. DO NOT SPLASH THE POT. When you put your bet in, move it forward, but not directly into the pot. Do not throw bets in willy-nilly.
There's plenty more to learn, but I feel like I've done my part, and I'll let my fellow cardschatters educate you further