I am a novice GM and this is my first time using Pathfinder. One of my players wants to be a druid but I don't know how to work him into the campaign. It starts in a typical tavern with the owner spiking their drinks making them think they caused a ruckus in the tavern the previous night. They repay the owner by clearing out his cellar for him. I don't know what the druid would be doing there. Please help. By the way, the other characters are a Human Sorcerer, Hobgoblin Fighter, and a Halfling Wizard if that helps.
You are losing before you win here. Why do you think it's your job as the GM to justify why a PC is there?
As GM you should explain the kind of campaign you're running and the starting point. Then you have the players make characters that will work well with that campaign and the rest of the group. You pose it to the player - "OK, Fernando, why is your druid hanging out in this bar?"
By having a setup that the players devise as opposed to one imposed from outside by the GM, that creates buy-in that ensures that players weren't always trying to bypass the perceived restrictions of a campaign or group conceit that the GM came up with.
Look at the Pathfinder Adventure Paths - they walk you through this. For example, in Curse of the Crimson Throne, you all have some kind of hate-on for Korvosan crime guy Gaedren Lamm. In Carrion Crown, you all like the now-deceased Professor Lorrimor enough to travel to his funeral. (They even make campaign traits to back up these connections.)
"Why would a druid be in a bar" - to sell hops, to get laid, to meet someone, to hide out from the other druids that want him dead, because he's an urban druid, because he is trying to hop a boat to the Savage Island... There's an infinite number of reasons. Why is this druid in this bar? That is for the player to decide.
As a GM the answer is really anything you want.
A better way to look at things is to worry less about what class he is, deep down he is just a person, and just like any other person he needs to eat, sleep, and socialize. Just because he is a druid doesn't mean he isn't allowed to be in a tavern, not allowed to drink, or not allowed to be outside of nature.
If you are looking more for an explanation of why the druid is in this particular tavern or this particular town, etc., that is something I would ask the player as part of his back story.
While other people here have provided excellent answers, I want to add that druids visit city bars because Mother Nature is an oppressively overbearing parent, and sometimes a druid just needs to escape to drink his/her problems away without having to brew the beverage themselves.
Also... The untamed wilds are not conducive to running a personal brewery, at least not for long. I'm never forgiving that family of boars for trashing mine mere days before I was planning to tap my creations... that habanero mead was going to be amazing. 6 skill points and a feat wasted on crafting potions that have yet to ever survive long enough to be used.
See what we did here? I came up with some backstory and had fun with it, and if I were making a character for your campaign, I'd feel closer to my character as a result. As the other posters suggested, give the players some campaign context, and then have them craft their backstory for why they're there. Put the onus on them, and you tremendously increase their involvement/investment in the game while reducing your own workload.
Maybe he's selling some hops to a brewer.
I would say that rather than shouldering the burden off onto one party or the other (i.e., the DM or player), the optimal solution is to come to some compromise. I know that, for me, as a DM, one of my favorite things to do is speak one-on-one with a player in order to hash out some reasons why they might be present in some location or another, why they might involved with the other characters, etc. I find that this sort of cooperative approach is a really great way to both flesh out their character, but also to build the setting and storyline around them.
To address your dilemma more specifically: You (and the druid's player!) have to think about what kind of role this druid plays in his environment and what kind of person he is. What are his motivations? His habits? Since druids are typically very naturalistic and concerned with the ecosystem, he may visit the tavern from time to time to speak with the community's hunters or farmers. He may have clandestine agreement with the innkeeper to play the role of a lookout on the community's periphery. He may in fact bring back some of the game that he hunts in order to feed the townsfolk if times are dire and he's particularly compassionate. He may have come this particular night to set eyes on a villager he's witnessed defiling a particular grove. He may also be wild, near-feral, but simply need to enjoy an ale in an isolated corner of the tavern from time to time to just to stay grounded. You see what I mean--the point is to step into the mind of the druid (again, with your player!) and explore what it is about him, his environment, and the tavern in particular, that might see them all together on a particular night.
Hope that provides some food for thought!