There's no easy way to start buying clothes. The best thing you can do is to try stuff on. Only then will you be able to tell what sizes you should be and what brands fit you best. Unfortunately, that either means being awkward trying on clothes in stores or spending a lot of money on clothes that don't fit properly.
Even when you know what size you are the variety in brands and even size differences within those brands will drive you crazy. I've just come to the conclusion that I need to try on everything before I buy it.
In general, you want the female clothes you buy to be more fitted than similar male clothes. Women's clothes are made to be snugger and tend not to look correct when they are too loose.
Regarding your pants question. It depends on the type of pant you are buying whether they are casual, dress, or jeans and how they are cut ie. high-waisted, low-rise, or standard. I can be different sizes depending on where the waist line lands on my body and whether they are a looser fit versus tight fit. As an example, I have some denizen jeans I really like. In the looser cut I prefer a size 8 but in the tighter skinny version, a 6 fits the best. Doesn't make sense that I went down a size but yay clothing sizes.
Good luck to you.