"interfaces like MAX or Maya, so I may want to get one of those."

Those are expensive apps ;)  You didn't mention budget.  I'd suggest a look at Silo if cost is an issue for two reasons:

A -- It's inexpensive at $109.
B -- The topology brush in particular is a strong tool for making clothing for Poser figures.  Check the forums at nevercenter.com for more specifics on the feature.  There 'may' (or maybe not) be copyright issues since you're using the base mesh as your 'template'.  I don't know for sure.  It's never been an issue for me since I don't distribute anything.

I'll toss in a vote for Wings if you want a free modeler.

As far as "Taking a course to learn modelling - for Poser clothes creation" there are some tutorials available for sale here.  I bought one by a vendor whose name escapes me that I was quite happy with.  It's lengthy because, well, there's a lot of work involved making conforming clothing.  Things are simplified considerably (as far as the creation of the piece goes) if you choose to make dynamic clothing.  I believe Phil C also has a tut for sale for making conforming clothing.  I haven't bought that particular product, but everything else I've bought from him has been excellent.