Attached or detached to the building with the main panel... it makes a difference
Lets just talk about a sub panel in the same building.
for a 120/240V sub panel you MUST have a (4) wire feed. That is (2) hot legs (1) equipment ground and (1) neutral.
The NEUTRAL bar in the SUB panel MUST not be bonded to the ground, ground bar or the panel box housing. It MUST be isolated on its own bus bar. This is a MUST.
You DO NOT have to have a MAIN breaker in the SUB panel, but it is a nice option.
The breaker feeding the sub panel MUST have both poles handle tied. This is a MUST.
The breaker feeding the sub (this should be obvious) must not exceed the rated ampacity of the service cable feeding the sub.
On the other hand, it is perfectly acceptable to oversize the feeder and still use a small breaker set to feed it.
The sub can be fed with copper or aluminum wire.
The sub panels physical location falls under the same rules as a main panel.
Now for an isolated building... things get a bit more complicated due to the grounding rules. Let me know if you need info on that.