I've been playing with Orcs and/or hostile locals for a while now.
First thing to do is build a ditch (you know, a "channel") around your starting area. This can get a bit complicated if you started on a slope, but mostly it gets done fairly quickly. Two thing to consider here: first; your dwarves will like to do jobs from the west or the north, so if you want to ensure that nobody will get trapped outside; leave a gap on the east or south sides and channel it out later. Second, to avoid your animals getting stuck, assign a meeting area.
It'll be a season or so before you need to break out of this, but once you do, chose your access point carefully. build a retractable bridge (3 wide is enough) over the ditch on that point and flank it with fortifications. DON'T FILL THE DITCH WITH WATER. I don't care what somebody said above. Water is a safety hazard. It can freeze in some biomes, and some things can swim through it. You're better off with a simple ditch. [seriously some people today....grumble....grumble...why do they say these things?]
You should wind up with a 3-wide entrance to your fort. Put cage traps across the entrance. Put channels for about 8-10 tiles straight out from your gate so that anyone coming towards you has to come by a single 3-wide corridor. Make sure that all trees are felled, all boulders smoothed within this corridor. Fill it with weapon traps. Wooden spiked balls (3 per trap), Green glass serrated disks, iron giant axe blades all seem to work well. After a few sieges you'll be able to make your own contributions from captured material.