You need to discover the map feature "magma sea" or "volcano". So you need to find a safe way to peek at the magma sea and then you´ll get the ingame message "you have discovered a great magma sea" or the like. After that you will be able to create magma workshops. Simply finding magma somewhere on the map is not enough.
As for the uniforms:
1. create uniform:
(m)ilitary - u(n)iform - (c)add uniform
select the new uniform with the direction keys (cursor up/down)
(n)ame uniform if you want to something like "hammersquad"
Then you can start adding items you want to be equipped. Example for a squad hammerdwarf (assuming you have tons of armor available):
(A)rmor - go to the selection list and add 2x mail shirts and 1x breastplate
Then to go the items, highlight them one after one and set the (M)aterial to steel (only if you have steel armor, of course).
(L)egs - go to selection and add 2xtrousers and 1xgreaves. Set (M)aterial of trousers to leather, greaves to steel.
(H)elm - 2xcaps, 1xhelm, 6xhood - caps leather or steel, helm steel, hoods leather
(G)loves - gauntlet, mitten, gauntlet, mitten
(B)oots - sock, high boot, sock, high boot
(S)hield - shield
(W)eapon - steel war hammer
Now press (R) so that the uniform replaces clothing (since socks and hoods and stuff are already part of the uniform and you don´t want your guys to get confused).
2. Equipping a uniform
In the (m)ilitary screen press (E)quipment and select your squad. Assign (U)niforms. Select the hammersquad uniform and press SHIFT+ENTER to set it for the whole squad.
Note that when you create new squads you can assign their uniform right during creation which makes life a bit easier.