Uuuuh, if you've -never- tried any women's pants on, it's going to be hard. It's hard to know to begin with, but if you don't even have a starting point... Sizes aren't the same at all, and it varies greatly. For example, my men's pants size is 30, maybe 28 or 32 depending on the brand. Doesn't vary much. However, in 5 pairs of women's jeans, I have sizes 6 (x2), 8, 9 and 11.
8: loosest; second worst fitting
9: second loosest; third best fitting
11: third loosest; second best fitting
6(1): best fitting
6(2): this one doesn't have enough butt space and is too tight on the thighs, so it keeps slipping down, is mostly uncomfortable and hurts my hips since HRT - it's actually my worst fitting despite being the same size as the only pair of jeans that really fits me
As you can see, how loose it is on me or how well it fits doesn't seem to have all that much to do with size. However, after trying things out a little, if I ordered online (which I'm actually planning to do this summer), I would probably go for size 6, since my bad fitting 6's are from a cheap store and are older. But even so, I wouldn't do it without fear of it not fitting. (By the way, all of those have been bought in stores, however my weight fluctuated during that time... and to be honest, for my worst pair, I just didn't check properly what happened if I walked or sit...)
That's why you should choose an online store to which you can conveniently return articles that don't fit without too much of a postage fee. Well, unless you buy super cheap clothes from doubtful sources, but that's another story.
And even in stores, it's not always easy. My sister took me to a store aimed mainly at teenage and young adult girls in which the very largest of everything fit either barely or not at all, whilst in other stores, I can choose among the smallest. Even with S / M / L sizes, it varies. I have loose Small pajama pants, and three barely fitting Large hoodies, one of which having actually ended up not fitting at all anymore because it shrunk in the laundry over time. It's not a torso - legs difference, since I tried Large pajama pants from another store, and I couldn't even pull them up.
So I really suggest that you at least do some fitting in a store to have somewhere to start from. Men's clothes sizes don't mean much if anything in this matter. If you're two embarrassed, do you by chance have a woman close to you (and whose clothes you can realistically at least fit in, living with you or not, who would be willing to shortly lend you a few articles of clothing to try on? It would at least give you -something- to work with. If you're living with them, you could, cough, sneak around a little, cough, if you're too shy or you think they wouldn't want to lend clothes. (Just make sure not to damage it, haha.)
