I'm sure some of the other guys here can guide you more thoroughly, but here's my suggestions:

1) the only requirement to purchasing a city plot is that either the president or a general (or advisor) have enough credits on them to buy it, and that you don't have another city already.

2) there is no limit or requirement for people to be present when buying a plot. We bought ours on the initial patch day when only one general & one member were online already.

3) after you purchase, your only immediate concern is placing an HQ within 48 hours. I would highly recommend that you have the HQ made in advance. The monthly upkeep won't be due for one month. I'm not sure if it counts from when you buy the land, or from when you place the HQ, but those should be very close together anyway.

4) the only other things I suggest are careful placement. The HQ is the largest structure you'll be placing (for starters anyway). If I recall correctly, the small HQ measures 3x3, and the meduim & large measure 4x4. If the land you select has a spot that a 4x4 building will fit in, you'll want to place your HQ there. It's also recomended that you make the highest ql HQ as you can to start. When you want to upgrade HQs, you'll need to destroy all the buildings you've placed, and start over, so the higher you start with, the better.

You'll also want to make sure you have is facing in a direction that makes sense (like, don't point the doors facing awayt from the center of your town unless that's what you want). This is true for all the buildings you place.

The only other bit of advice I'd throw in is for placing your city grid building. If you can, you'll want to place this as far away from your city controller aas possible. If your grid point is too close, you'll have problems with getting aggroed by aliens when you grid to your city in the middle of an attack. You'll be much better off spacing this out.

If you need any help with items for making buildings, or tradeskills to assemble, sent me a tell in-game & I'll be glad to help ya out. I'm quite a packrat, so I've got plenty of spare city-building items. There's also 3 empty city plots out where my guild has ours built. I think one of them is pretty small, so let me know if you'd like to come check it out, since more neighbors would be a good thing.