Kibstable has the right idea, but isn't quite the best plan.
The most reliable plan for an initial defense is to dig a 1 z-level deep moat, and remove the channels from all but one tile (the "entrance"). The zombies won't intentionally drop down the "cliff" and you then have time to get safely underground.
If you bring two miners with at least Competent Mining, it shouldn't take more than a day, game time, to dig the channel and remove the ramps. If you remove all the ramps, then dig an upward ramp into the center you'll be able to get them out, but will have a way for stuff that falls in to get in as well. Not as bad as it sounds if you brought some wood (or better boulders to be turned into blocks) for a wall. An 11x11 moat would allow a wall requiring 38 blocks (10 stones). Rock Craft quality stone is usualy 3 pts per boulder, so 30 embark points for 10 boulders. Add some wood (or deconstruct the wagon) to build the mason shop, and crank out the blocks while the miners dig the moat. Four hammer dwarves to provide close support and you should be able to secure an 11x11 tile area of the surface in relatively short order. Don't forget their shields.
I haven't actually tried this, however, so I may be missing some bit of info that makes this more difficult. It is, however, what I would try in your situation.
Oh, and bonus Dwarf Points for the dwarven-tenacity to stick to your site and get a fortress built. Once you manage it, you will be justifiably proud of your accomplishments.