Platinum does make for good war hammers, but last I checked you can only make platinum hammers through a strange mood.
My advice is to set up traps (stone-fall is easy. and should do the trick for the first siege or two), and also an emergency burrow (encompassing the farm, still, meeting hall, and maybe any levers you might need) which you can use the civilian alert to keep your dwarves in should things go badly.
Also work on getting a full-time military. At least one squad should be fully outfitted with, at minimum, a breastplate and helmet as well as shield and weapon. But everyone who fights should have a shield and weapon. When it comes to armor, it's kind of ok to skimp and use wood and bone...but I wouldn't recommend it. Ensure that all weapons are made of metal even if it needs to be copper.
Personally, I tend to put my entrance at the end of a canyon and then remove the surrounding ramps. You may not be able to do this, but you can still set up ramparts and the like if you so desire. I actually recommend you set up walls or something along those lines, with a bridge leading out of them and a moat around the side. Within the walls should be a pool of water with a raised well.
Get a sustainable water source and a well ASAP. Also, you should look into setting up a hospital. If a dwarf survives a few battles, he may well become a capable enough warrior to survive many more, as live combat significantly raises their skills.
The link I edited into my last post also has some helpful information on construction.
EDIT:
Make exactly one charcoal at a wood furnace, and then turn all your coal into coke.
From there, smelt all the tetrahedrite. All the copper bars you get should be turned into edged weapons (axes and spears. Swords can be nice, too) and breastplates/helms. The few silver bars that result should be turned into warhammers. Use wood for shields, unless you've got extra copper laying around.
Once I've got the necessities for EVERYONE in your military (so 30 or so sets of weapon/shield/breastplate/helm) I start making leggings, then high boots and gauntlets. But they aren't strictly necessary.
It might help to get a cloth industry going, and then making a lot of cloaks. Equip your military with cloaks. I don't know why, but those seem to really help.
EDIT2:
I didn't see that you had iron.
If you have flux, make steel and then make all your armor out of that.
If you don't, make all your armor out of iron. If you have any left over, make your edge weapons out of iron as well. But copper is fine, too.
While it's true that offense > defense in DF, you still want to try and keep your dwarves as well defended as possible when they go into battle and sacrificing good weaponry for good armor is good for this.
If you smelt bars and then make your equipment, you can count to see how many of each type you have. This could let you use the copper to train an armorer's skill up, and provide better quality iron armor.