What do I do? Do I tell the cops everything?

for one its not as simple to call cops and say hey my step father touched you when you were little. There will be a lot of hle of investigations etc.... If he had with you or put his hand down your pants then yes def and give them all information. Your mother will prob get arrested to but if she knew then she should know better. This might make you feel better to know that its being dealt with and that you don't have to worry anymore. Then get on with your life. Sexual ault is a big NO NO for kids esp because we are innocent and don't deserve to be taught such things when we are that age. Hope this helps