Grab both clothes from the men's and women's section. Only get the sizes you think will fit you in the gender you need and if you want a general guess about where you are in terms of sizing, some websites do conversions of male-to-female sizings and female-to-male sizings. Shirts can be pretty easy as it's usually: X-small, small, medium, and large or beyond. Go larger on those than if you were picking guy attire, so if you wear medium in men's go for large/x-large in women's unless they are androgynous cuts or styles of shirts. Be mindful of the cut or style of shirts, some cinch the waist more, have dipping v-necks, or have more feminine styles that make the shirt smaller than it states on the tag.
For instance, a "babydoll" shirt has an hourglass cut which makes it tighter in the waist but flares on the bottom so despite me being a medium/large in regular women's shirts, I often have to go for large/x-large if I go with a "babydoll" shirt. (I'll admit it's due to having a gut, not other oversized proportions.) So don't be shocked if you don't always fit one size as brands and styles vary across the board. Just take the first step and you'll be a natural before too long. Don't be ashamed to browse the women's section, just act like you belong there and people usually leave you alone.