I’m going to run a new campaign, but I don’t have a way to introduce them to the BBEG.  I’m thinking of having the first fight be a battle between them showing how powerful they are and leading to an inevitable loss.  Then they will wake up in a inn wondering what happened.

I mean what’s their overall deal?

Why are they fighting the party, what motivates them? What motivates the party, why would they put themselves between the world and this person?

Like level 20 is godslayer level, if they fight that at level one it’s over on turn one and then you have started your campaign by making everyone fight a fight that was inevitable and few bummed they lost.

the first couple fights can be important for a party, they are feeling out new characters and abilities and what play style to attempt. Does a fight they can’t win hamper that.

personally I would shelve the Druid for now and see what characteristics your party develop to make your BBEG a reflection of or counter to.

have them be an unnamed force in the background opposing the party through coincidence more than intent.

let your party get confident and strong and too full of themselves and then have the Druid humble them. 

Introduce the party to a powerful and friendly NPC. Maybe for fun... play them as a stereotypical DMPC (clearly the hero of the story, interjects themselves into the party's business whether they want him to or not, keeps solving problems that someone else in the party could have handled). Then, just as they're getting annoyed with his antics, the BBEG and some minions attacks the town everyone's in. DMPC rushes headlong into conflict with BBEG, while the party is given something to keep them distracted from attempting to help directly (maybe there's a group of villagers they can focus on protecting). They watch as DMPC barely stands a chance against this guy and gets his head smashed like a watermelon at a Gallagher show. The party gets a strong sense of how strong BBEG is without serious risk of a TPK. You get to toy with their expectations a bit by setting them up for one scenario before pulling the rug out from under them. BBEG wins, casually dismisses the other party members as not worth his time and takes off, leaving the party to clean up the aftermath.

Did you ever watch the first season of daredevil on Netflix? No one even says the big bad’s name until the 3rd episode (of 10).

I know it’s tempting, but keep the big off the board for a while. Let the party deal with one of their minor flunkies for a while, thinking the flunky is the big bad. Only when the defeat flunky do they realize there was something else pulling the strings. Even then, they might only hear the name of the big bad, but probably still need to work through another layer or two of the organization before they actually know where the bad is, and the true nature of their plan. Keep the mystery as long as you can. Tease them with bits of information, but the whole picture shouldn’t resolve until late in the campaign. 

I was thinking of having a cult that follows him, he has a god complex, that have advanced technology, have them with a new/homebrew type of magic, or both.

Btw he is lvl.15 Druid and a lvl.5 Fighter.

I was thinking of having a lvl.10 wizard or fighter that saves the party after their first encounter with BBEG or his cult.

They’re fighting the party because them and a few other groups of good guys are trying to interfere with their plans of the cult.

Thank you all for the suggestions they really helped for background and molded the world nicely.

It sounds terribly railroady to me.

Starting with a fight you can't win, getting beat, then getting saved by an NPC? More railroady. then just mysteriously waking up in a tavern. How'd they get to the tavern? Is the wizard going to be hanging about? 
If I was playing in this game, I'd feel like taking a beating is a pretty bad way to start things off. And then getting saved by an NPC would just feel like I'm along for the ride, rather than dictating anything.

So far, you have your (the DM) big bad win the battle, then your (the DM) save the day with your NPC. It sounds like you'd rather be playing than DMing.

Figure out the plot and create a hook for the PC's to pursue, not just beat them over the head with it.

I prefer Xalthu's solution.

It offers several advantages:

1. You can design the minion to be just barely beatable by the PC's, without any fear.   If he wins the battle, he can be called away before he actually kills the party.  If he loses the battle, he can be rescued by the real BBEG just before he is killed.    This lets you avoid railroading the PCs
2. As speech is 'free', you can have him do a classic super-villain speech as well, just before he leaves "But I can KILL these bastards ... " YANK! teleported away. 
3. It gives the players some knowledge, but not a lot.  Make them work for it.
4. You do not have to actually write up the real BBEG yet.  Save that for later once you get an idea of how the PCs fight.
5. You have a Minion you can use repeatedly - either alone or to help out the BBEG once the big fight actually happens.
6. You have someone you can let the PC's actually kill in a later battle, that will feel significant rather than just another fight.  "We finaly killed that SOB!"