There are many many kinds and causes of rashes, but for the most part (mermaid thighs being a notable exception) if you go to the doctor they will all treat it the same way at first. This is what they will do:

1) Benadryl for the allergic reaction. One pill every 8 hours. It can also be rotated with a daily allergen fighter like claritin or zyrtec, but the benadryl will be used until you can work up a therapeutic dose of the other kind.

2) Keep the area clean, cool and dry. Heat and sweat can exacerbate rashes - whether they are hives or heat rash. Sweat can also be an irritant. Avoid all products that could cause or increase irritation with the exception of:

3) A gentle, mild soap used at least once a day.  My current love is this french pharmacy brand but really anything that is marked for sensitive skin will work.

4) A clean moisturizer, also marked for sensitive skin.  For the face I like Atopalm, which is also awesome if you have some kind of irritation from trying a new product that isn't right for you - Alicia Yoon from Peach & Lily turned me onto it.  She used it when she had a bad allergic reaction right before her wedding and the results she shared on Instastories was magic. For the body there is nothing like the magic of pure lanolin, which is extremely hydrating but also non-greasy and gentle, like this all-over everywhere cream from Lanolips.

5) A steroid cream.  Pictured here is the prescription one they always prescribe, but plain old OTC hydrocortisone is usually my first stop. Although once you get this one it's hard to turn back, if for no other reason then even as a scrip it's big and cheap.

6) A find dusting of powder for high sweat areas. I try to avoid this, but if the heat is inescapable, or the rash is somewhere that just sweats (like armpits, cleavage) a fine dusting of plain, baby-powder can help keep the area dry and sweat free while it heals.

See that little patch on the back of my thigh? Mystery rash, what caused it I may never know. Fortunately the treatment is usually the same.