All the nuances of making and equipping uniforms is, like, a lot of steps and details. I was going to link you to the wiki's walkthrough (well, here it is anyways:
http://dwarffortresswiki.org/index.php/DF2012:Military_interface) but it's.. not so organized for just learning how to do one thing.
You've already made a uniform by pressing (m)ilitary -> (n)uniforms and using the interface there? Apply it using (m)ilitary -> (e)quipment -> (U)assign uniforms. Scroll vertically with the up and down arrows keys to select a squad, then press the right arrow key twice to get over to the "uniforms" tab, select the uniform you want to use for the squad and press shift-enter to apply it.
Press (V) from the equipment screen to go back to the regular equipment view, where you should now be able to look at your dwarves and check that they all have the uniforms you just assigned them. Unpause, run time for like a day or two, then check again and there should be a green symbol to the right of each item in each soldier's uniform showing that they've successfully acquired one of whatever item it is. (I'm using Ironhand's tileset and I don't know what the symbol for equipment check-list is by default.)
From the military screen, press (f) for ammunition assignments, scroll vertically to your squad, and press (c) to add a type of ammo. By default this is bolts, so just press enter, and adjust the amount with + and -. I feel pretty weird giving advice about ammunition because right now I cannot get my marksdwarves to pick up their goddamn bone bolts, BUT USUALLY IT WORKS. ok.