I took my band from a 5-piece, ~$500 a night bar band to a 6-piece ~$5000 a night wedding band about 10 years ago. In the process we lost one of the original 5 musicians who didn't want to go on the journey and added 2 female vocalists.
Your main competition is going to be from DJs and the trick to convince the client live entertainment is worth the extra money and then you (of course) have to deliver on the promise.
A lot can be learned from the DJs -- give 'em the familiar party hits, keep it high energy and upbeat, don't bore them with a lot of long guitar solos or space between songs. We have found younger audiences have short attention spans so we do a lot of medleys where we run though as many as 10 songs in 20 minutes. But these are not the types of gigs where you want to start playing songs that you really like that maybe they don't know because you think it's really cool.
Some of the stuff said by others is very true. Including don't undersell yourself. We turn down gigs below a certain price. We are going after high-dollar clients who are spending a LOT on their wedding. They actually expect to pay more in many cases because they understand with a higher fee comes a better product.
Be prepared for long days. A typical wedding might include needing to be set up by 3 PM before they start their ceremony, and the band won't start playing dance music until 8.
We upsell additional services rendered. We go in with a base price for 3 hours max of reception/dance music. If they want us to play during their cocktail hour, or during the ceremony, or provide recorded music in a separate location, or they need a wireless mic for toasts and speeches? Those all get added onto the price.
We carry around enough gear to accommodate those various situations. Powered speakers. Wireless mics that can be used by guests or the officiant. A small mixer in case we need to set up in a 2nd location to do a cocktail set.
You'll need a dedicated sound engineer.
You'll need lighting.
The boys all wear matching suits. Small thing, but the clients always seem to appreciate the professional appearance.
We offer to learn up to three 'special dances' --- Bride & Groom and parents' dances. We usually have fun with this and enjoy the challenge, even if the songs often only ever get played that one time. Pro tip: they don't want to dance more than about 2 min. Get out before the bridge. Less to learn.
The setlist is pretty time-tested for effectiveness. What I've found works best for us is we start the night with some disco and Motown classics that virtually everyone knows, move into some 90s/00s and a couple of current songs that maybe the older folks won't know so well but the younger guests will, and then end the night with some classic rockers everyone can stomp and sing along too.
One thing different from doing a bar gig where you play to the same fans regularly -- we rarely play for the same people twice. For better or worse, our setlist hasn't changed THAT much in 10 years. No one gets burned out on it. The more current songs need to be switch out every year. Current hits fade quickly. Although we've been doing this long enough that now we are bringing back a few of what used to be current hits as 'classics'. lol
We work on getting the crowd involved. Including bringing people up on stage to dance around and sing along and whatever creates more fun for them. Stuff a DJ can't do. #1 rule? The event is about the bride and groom and their friends and family. It's not about the band. We aren't there to put on a concert. We are there to make their wedding pop and be as much fun as they can possibly have.
You'll need to hook up with agents if possible and have good promo material. Professional photos and video. That can cost you some money up front.
All n all, it can be a lot of work to run a band at the level you'll need to run if you want to make decent money. And a lot more than my band charges can be earned by bands who offer full horn sections, dancers, etc. Find the niche you're comfortable with and work it. But it's not for every band/musician. Many would much rather just show up to an easy gig, set up their amp, play songs they mostly like, and get their $75/per man and drink tokes and head home. Nothing wrong with that. What we do and how we do it pays a lot better but it IS a lot more work. So you gotta be up for it.
Best of luck on your mission, should you choose to accept it.