Lots of anti-dandruff shampoos can be used for jock itch. You want something that has selenium sulfide or zinc pyrithione in it. The higher concentration the better (as long as you're buying it off the shelf in a grocery store anyway).

Apply the shampoo to the affected area. Leave it to sit for the time stated on the bottle or longer because that really is not going to do you any harm. If you really want to knock it on its head apply the shampoo before you shower so it doesn't get diluted by the water and let it sit on your skin for a few minutes and then jump in the shower.

Rinse off. Dry your skin thoroughly, paying close attention to the affected skin areas. I'd recommend using a hairdryer or a fan to dry your skin in these areas completely before you get dressed. Repeat this daily until the itching and skin discolouration is gone then continue with the treatment for at least a couple of days longer.

Keep the shampoo bottle in your shower and if you notice any itching starting up again, apply another round of the shampoo as a maintenance thing and you should be good.

If this doesn't work then see a doctor/speak to a pharmacist and bring out the heavier stuff. But you might be surprised at how effective antidandruff shampoo can be and you can get a huge amount of treatment for only a couple of dollars. Probably less if you can find antidandruff shampoo in the dollar store where you are.