first off - well done getting a gig!
that means someone got confidence in you and your ability.

be as professional as you know how to be. make a plan in your minds eye as to what you'd like the gig to be like. write it down, as much as you can the song titles and the order you want to perform them. make a note of the kind of rhing you want to say to introduce them. remember people don't know much about the songs. look them up, and the writers or their history as trad songs. if you know somerhing that strikes you as interesting tell the audience - they will find it interesting, hopefully.

get to the venue in plenty of time. find out how long they want you play for roughly. wear a watch. its bad manners to keep looking at it onstage. but try and do what the organiser wants time wise. if theres a PA system try it out before the audience is present, if possible. if you use an string instrument - get it out the case and let it acclimatise itself to the temperature in the room. digital tuners are there to help us.

try to vary the   songs so that there is a sort of flow to the evening.

and finally most folk club audiences are great, but there will be people in the audience who reckon they should have got the gig. you have entered a field of endeavour which is so competitive it makes premier league football look like the teletubbies. not everyone will love you. get used to it.