Create a militia. Militias are made of squads, which are made of dwarves. You must designate one dwarf to be your militia commander. You can do this from the {n}obles screen. Once you have selected a militia commander, press {m} and then {c} to create a squad. You'll be asked what uniform they should wear. Press {Down} until you get to "metal armor", which is your best generic melee-dwarf uniform, then {Enter}. If you wanted this to be a ranged squad, well, that'll take more keystrokes.
Recruit 3 dwarfs. Your militia commander will automatically be put into the squad. So far so good. Now, press {Right} and {Down} and you should be on the first "AVAILABLE" slot under Squad Positions. Press {Right} one more time and you're on the list of unrecruited dwarves. Scroll {Down} to the dwarf you want to recruit, and press {Enter}. Now you've got 2 dwarves in the squad. Press {Left} to go back to the Squad Positions column, and {Down} to go to the next "AVAILABLE" slot. Press {Right} to go back to the list of unrecruited dwarves, {Down} until you get the one you want, and {Enter} again. Now you have 3 dwarves in the squad.
Equip all 3 with axe, shield, armour. If you went with the "metal armor" uniform up front, then this step is automatic and requires no action on your part except the acquisition or production of the equipment itself. The dwarves should start performing "Pickup Equipment" by themselves.
Tell them to guard a spot. This is where it gets much more pleasant. Press {Escape} to go back to the game if you haven't already. Move your view to the place you want them to go. Press {s} to bring up the squad menu, then {a} to select your first squad. Now press {m} to issue a move order, which gives you a yellow X cursor. Move the X to where you want the squad to stand, and press {Enter}. You'll see the word "Station" flashing in green next to the squad's name. Press {Escape} to get out of the squad menu. The dwarves will stand near the spot you selected until they die, basically. (But they will attack enemies who come near.)
Holy shit, they're starving to death! You have to cancel the "Station" order. Press {s} to bring up the squad menu, {a} to select your squad, then {o} to cancel their order.
If you want them to guard a spot long-term, but still eat and drink and sleep periodically, you'll have to look into the more advanced stuff like patrols and waypoints and god knows what. I haven't bothered with that yet. I've been able to get by with just station/kill orders and some training.
Good luck.