I want to lose weight...10 points?

I'm really desperate to lose weight around 3 stones in 6 months, so I'm going to get a gym membership. I'm hoping to go at least 4 days a week along with eating healthy. I've been to the gym before but i didnt manage to lose much weight because i never really had a workout plan i was just going from machine to machine not knowing what exactly I was doing :/ so this time I'd like to know how to workout to lose weight at the gym ? What machines are best and how many minutes I should use each machine. Thank you so much to anyone who answers :-)