I have a dress where the material is really see through in the sun. You can see the outline of everything. the dress itself is a dark color and the material is cotton. What can I wear under it so it's not see through?
You should wear a full-body slip underneath that dress. It won't stop the dress from being see-through, but it will help conceal some of the parts that are outlined and provide a more dress-like silhouette.
If the dress is cotton, get a slip made of a silky/satiny material. Read up on what type of acrylic blend you're buying. Some are not really great to use for a variety of hygienic reasons.
If the dress is a silky/acrylic material, you can opt for a cotton-blend slip, which is best for the summer months.
Can you see detail or is it just sheer enough that you can make out the outline of your underwear? If that's all it is, then try just wearing underwear of the same color as the dress. A gstring or thong would also be less noticible. If you can make out detail, then go with a full slip.
I usually wear a thin skirt - a bit like a petti coat. I got it from my mum so I've never actually seen it in the stores ha ha (maybe she hand made it to subtly tell me to cover up). I have plenty of sheer skirts and dresses and I wear that underneath it.