Getting back into it

I've been away from MFP for a while, and due to some health concerns that forced me to take steroids, I gained a lot of weight this year. Currently living in a group household where we generally cook together, which can be really nice, but not great for weight loss! Any tips on how to ensure I don't go over my calorie limit? I am certain I am currently underestimating and I find it difficult to figure out calorie content, especially if I'm not the one cooking.