I need help with trying to send one of my friends a trade. I don't know how to send it to her. Can someone help me please?

Is it an item? Click on the item in your inventory and then click the small envelope icon below your inventory. This is the mail button. It'll send items to another player.

If you want to send money you'll have to ask your friend to send YOU an item with a CoD (found on the mail popup that appears when you press the envelope button). A CoD (cash on delivery) is the amount that you must pay to receive an item from another player. So they're basically selling you a cheap item for whatever amount it is you want to give them. Players often use mushrooms for this purpose and you'll sometimes see players ask others to "mushroom the money". Meaning to send them a mushroom with a CoD.

Thanks so much!