Embark with enough leather or preferably metal or ore to completely outfit a number of dwarven warriors. I prefer bringing cassiterite and malachite/native copper since they can be smelted into bronze from ore cheaply and provide acceptable armor, and copper makes cheap training weapons. Make sure to bring some iron for fighting weapons. Fuel can be made on site. For skills, pick four dwarves and invest in their shield and wrestling skills to help minimize spinal injuries in training. Make sure you have an even number of military dwarves so the odd man isn't standing around. Give them a mix of blunt (for stun) and axes (for limbs). Also make a number of crossbows and bolts (out of bone or wood if money is scarce - you want them for criticals, not damage) so that your three starting civilian dwarves can also fight, and bring as many dogs as possible. Your first year will be pretty rough with a limited work force, especially because you'll be sacrificing a lot to equip and train your military. Skimp on bringing food and booze and gather as much as you can from the countryside. Alternatively, organize your military to take down the local fauna and save the flora for fermenting. Have your miner or mason try to construct multiple choke points through which the orcs must travel so that they trip over one another and get strung out, then attack the head of their column at the last one. If you're lucky and they attack in the first winter instead of the first summer, you should have four weaponlords, some normal weapon users (drafted from immigrants with leather armor), a bunch of war meatshields (dogs) and their meatshields (their pups), and a number of drafted civilians with crossbows. Don't even think about starting an actual fort until at least your second if not third year.
Oh, and the "friendly" orcs on reclaim are kind of a mixed blessing. They will kill some animals, goblins, and kobolds for you. Even then, it's probably a good idea to wall them up in spring. Interestingly, they don't seem to path to a dwarf unless they see it, so orcs sitting in isolated areas can be ignored. If there's one in your fort, though, be prepared to station at least two weaponsdwarves near it to ensure it dies if you get sieged that season. Dwarf meat must be like Prozac to them orcs.