I find 3-4 dwarf squads work best, keep them training constantly. Weapon varies by preference (always pick the best material for the job; silver hammers will demolish poorly armored targets but can often get deflected by bronze and iron armor, whereass a steel sword or axe would work best against large or particularly sturdy foes) and wood shields are advised to save on weight. I find a sane uniform works best, providing adequate protection with a minimum of weight, and even copper is better than nothing. Using the current DFhack will also make training soldiers considerably easier as it fixes a whole bunch of problems with it (equipment storage, training times, stuff like that.)
Metal bits: A must to minimize injury without hampering speed too badly
1 Helmet
1 Breastplate
1 Mail shirt
1 Set of Greaves
1 pair of gauntlets and boots
Nonmetal bits: More extra if you have the supplies or want to add it anyway.
1 Cloak and hood
1 bodywear
1 pair of pants of some kind
1 pair of gloves
1 pair of socks
Preferably, these would be made of leather. Layers and layers are all well and good but a good soldier needs to be light on his feet; he can't rely on his armor to stop everything.
For marksmen, speed is everything. Until they're trained up sufficiently, I personally go with this for ranged troops to maximize their rate of fire. Shield is optional but should be made of wood to save on weight.
Metal bits
1 helmet
1 mail shirt
1 pair of gauntlets and boots
Nonmental bits
1 cloak and hood
1 leather bodywear
1 suit of leather armor
1 set of leather leggings
1 set of gloves and socks
Weapon is one wood or bone crossbow. Again, this is to save on weight and increase thier rate of fire and save resources for other things. Make sure the squad also has ammo. I recommend assigning bone and wood bolts for both combat and training, and metal ones for combat only.
Marksdwarves will train at targets that they're assigned to train at that is inside a main barracks defined from a box, stand. or rack. Melee troops simply need the main barracks set up. Just an armor stand or bag/box set up and defined will be enough. The squads do need to be assigned to the barracks to train there though, as otherwise they'll sit on their thumbs in the meeting areas.
Embarking with at least one soldier is often a good idea. Some opt for novice weapon skill, and boosting something else like dodging or shield and giving them proficient teacher. I personally go with novice weapon skill and competent in dodge, shield, and armor use so if need be they can fight off early threats and fight decently against animals alone.
I could say more but my sleepiness has me worried much of this is unintelligible and BlackFlyme ninja'd me in some stuff.