Relax & don't worry about it. First, I'd be shocked if you ever come up against hecklers - I've been playing nearly 40 yrs and never have! Audiences are generally very forgiving, preoccupied with their own conversations, and chilled from a few drinks. When we make mistakes, we know...but the audience hasn't got a clue. If you get hecklers...just ignore 'em - if you're doing 2) below, you'll get the audience on side anyway.
Top tips -
1) if you make a mistake - just KEEP PLAYING...cos that's what the pro's do! And the audience will never know anyway if you missed or repeated a verse or got the lead solo wrong or played it in the wrong place. It's all what makes playing live fun & interesting. You'll have great gigs where you play everything perfectly, but the audience may not have been that attentive. Then you'll have gigs where you mess up some songs....& the audience will think you were all terrific!

Strange but true, I promise you.
2) Look as if you're enjoying it & engage with the audience - smile, and be a bit animated ie not standing there looking down, unconfident, and static. Folk are there to have a good time, so don't just play...entertain and engage with them at least a little.
3) Link the songs with some intro banter (helps engage with the audience & gives time to change settings, patches, quick tune up etc - & introducing a song avoids mix ups for the band as in 4) below!) rather than just merge one song into another.
4) Write out a song list and make sure everyone has a copy - you don't want to lose your place and all start playing a different song (see 3) above too).
5) Take a back-up amp/MFX unit and have it set up on stage in case you need it. Just knowing its there as back up for any equipment failure will make you more relaxed & confident!
6) Even if you forget a lead solo - just improvise....or if you have a mind melt down & forget the key, just lay back until you get into the groove again. The audience will never know!
7) Your drummer should be leading you timing wise into each song, so get into the habit of letting him count you in...if it's good enough for top pro bands....!

And MOST IMPORTANT just - RELAX, SMILE & ENJOY it!

"I started out with nothing ...and I've still got most of it left!" (Seasick Steve)