I want to create a small town somewhere in the Wasteland, how do I start?

I'd follow the tutorials on the GECK wiki first to familiarise yourself with the tool.

The thing with modding the Wasteland is looking for conflicts with other peoples mods. Say you go to map a town down near Superduper Mart, you put the file up to download and someone downloads it who has another mod running which places a big castle or some such on the same spot.

It would only be for my use and Im familiar with some of the features of GECK.

Okay, well knocking up a towns a pretty big feat. It's the same as you'd do any other mapping in the GECK, finding the right kit pieces etc.

What is the easiest kit?

There is not really an 'easiest kit'. Just a bunch of them and you figure out how they fix together. The easier kits are the indoor ones, but the outside ones arent too bad. What kind of town do you want it to be? Like megaton with the rustic metal shacks everywhere? Or like a city with actual buildings and such? That decides what kinds of kits you will use for it.

Start small. Make an interior like an inn or a house and link it to the outside world. Working in the Wasteland can certanly be a chalange but also very inspiring if you build a few things. If you just mess around and build some walls for your town/city, create a story for it, who the people are, what the town does (merchant pitstop, safe haven for runaways, someone just wanted to settle down.) once you create things in your head, you want to make it real. And that's your insperation.

If you have absolutly no idea of what you want, download the RTS mod for Fallout. Build a little town and see what happens. After a while, you'll want to expand beyond what the mod offers, so that's when you pop open the good 'ol GECK.

A good way of figuring out what pieces you might want to use is to open other cells and look at them. You can double click on a pre-built piece to learn its name then find that piece in the Geck and drag and drop it into your city cell.