How many Dwarves can you spare? For your first military I reckon a quarter to a half of your population as soldiers should do fine.
In your [n]obles screen elect a militia commander, preferably a Dwarf you do not intend to fight with, at least until you are confident enough in micromanaging your army later on, to avoid having to worry about finding replacements should your commander die.
With the militia commander appointed, in the nobles screen appoint your first militia captain. Then go to the [m]ilitary [p]ositions screen, and while highlighting the militia captain title [c]reate a new squad. I generally like having large amounts of captains, so create squads of three. But to get the hang of things it's better to first simply create squads in upwards of 7-10, by selecting Dwarves from the list on the right while the position you want filled is highlighted. The top right shows what squad, if any, the Dwarf is already in so you don't accidentally misplace Dwarves across squads when you're recruiting new soldiers.
From there go to the [e]quipment screen and assign [U]niforms to your Dwarves by highlighting the squad position and assigning it the uniform on the right; you can also assign the uniform to the whole squad with [shift] + [enter]. After that's all done, build any coffer, cabinet, armour stand, weapon rack or bed and designate it as a barracks from which your squads can train, and go back to the [m]ilitary [a]lerts screen and put your squads you want to train on [active/training]. This won't get them training just yet, so go to the [s]chedules screen from the military menu and give your [o]rders. Give a training order for 2 squad members for every 3 Dwarves you have in the squad, or just for every pair minus 1 [for example, a squad of 7 would have 3 orders for 2 dwarves to train while a squad of ten would have 4 orders for 2 to train]. This is to ensure Dwarves also have access to off-time for drinking and civilian work.
Once you get the hang of that you can start experimenting with squad equipment, uniforms, orders and barracks.