I agree it's best to plan ahead. Having a generic outline of the town will help you a lot even if you choose to change things on the fly later.
What's the population of the town? I've been in a lot of small towns for various jobs and, at least in my experience, there's a breaking point where the town goes from being a "satellite" town to a nearby larger one where everyone commutes to for work to being a self-sustaining town in its own right. Of course farming communities and logging/mill/oil towns will be their own centers of work.
What kind of town is it? A farming town? A mill town? A sleepy residential satellite to a larger neighbour? Mountain towns usually are resource based, having a mill or mine as the main source of work. They are also easier to cut off from the outside world with less roads accessing them, not withstanding all those forest service roads (useable only by 4x4s or ATVs) criss crossing the boonies. Farming towns tend to be more spread out (in my experience) and have easier access.
Also, I've found that small towns have some of their population spread out in obscure little roads with a few dozen homes intermixed with woodlots or farming acreage.
Town centers often consist of one or two main drags with other main streets intersecting them. Smaller streets shoot off from these with more houses and businesses. Quite often the middle of town will be laid out like a grid. Geographical boundaries will mix this up (hills/bluffs, rivers, marshes, a lake).
Found on the main streets or just off them will be:
Gas stations/Corner stores/Restaurants/Garages/Hotels & Motels
Retail shops: If the town gets tourism at all you’ll get more knick knack shops selling curious. Self-owned shops usually can’t afford the rent on the main drags and are off a bit.
Banks: Pretty much always found in the main part of town
Possibly larger stores: A lot of Wall Marts have moved into small towns (pop 5000+) or you may have the local co-op selling everything from soup to nuts.
Drug Stores/Pharmacies
Used Goods: selling old books, old furniture, clothes – every small town has at least one
Outdoor Outfitters – if the town is a jumping point for campers there will be several. If your campaign is in the States (like most) it would sell guns and ammo as well.
Churches (not as often on main streets but they can be)
Bars/Pubs
You could have a small office building including offices for the local lawyers/notaries/accountants.
If the town is big enough it may have a strip mall including some of the above
Community services: Police, Fire, Postal, Hospital or clinic, Town Hall In my experience these are usually a block or two off the main streets. Don’t know why, maybe the rent is cheaper.
Schools will be in the more residential areas.
You could include a car/truck lot, new and/or used. Possibly even RV sales. These would likely be on the edge of town though not always.
It could have a radio station. The local tower would be somewhere up high but the station would likely be in town.
If the town is resource based you’ll find large garages for servicing farm or logging equipment, plus more specialized shops (seed stores, chain saw sales/repairs, tire stores, ATV sales/repair.
Franchises have crept in everywhere. If the town’s really small you may not have many but don’t be afraid to throw in McDonalds, Subways, Esso’s etc) but that’s for flavour so it’s up to you.
If the town has a nearby river (most do) that will influence how you lay things out as well. If there is a highway going through the middle of town many shops (especially gas stations, restaurants and hotels) will be along that. It may even have a full blown truck stop.
There’s enough variance that you can ignore any of the above and mix it up if you feel like it. You could have a river with a bridge running through the middle of town, splitting the services.
Really small towns have very little in my experience. A gas station and local general store (sometimes one and the same) can be all sometimes. These towns would likely be within half hour of something bigger though.
All the above is based on my anecdotal experiences doing seasonal work throughout rural areas in Western Canada for years so others may have vastly different opinions.
I googled “small town maps” for images. I suggest you do the same, one of those should inspire you to do a basic layout of the town then just start dropping in all the notable stores/facilities you want in the town. Just don’t forget to put in houses
.