Transition

I have a 55 gallon long. I currently have gravel, however I'd like to change to sand. I've never had sand before. I don't know what kind I should buy, what the process would be in switching out the gravel and adding the sand, and I don't know what to do with my fish during this transition. I have 3 adult swordtails, 5 baby swordtails, 3 platys, 2 guppies, 4 killifish, 2 dwarf gouramis, and a bristlenose pleco. Any help would be more than appreciated. Thank you.