When looking at a new clients nutrition and needs there are a few basic steps you want to address. The first thing you want to do is look at what/how they are eating now. You can do this by having them journal for about three days without judgement. As far finding out what their actual caloric needs are, unless you have access to a machine you can only get a general answer to this question. There are a lot of calorie calculators on the internet and for the most part you are going to get the same results. When looking at making changes to their diet remember not to go too far away from where they are today. The reason for this is most people will not be able to sustain such a dramatic lifestyle change too quickly. Take baby steps if they drink 6 sodas per day for example you can’t expect them to go cold turkey and drink only water. Reduce their intake to 2 sodas per day and so on.
As far as the exercise component take it somewhat slow. When beginning their exercise regiment you want to focus on movement education. You will also want to have them become a generally more active person doing things like parking their car a little further from a place they want to go, getting out there to walk more often are good examples. You want to get them doing resistance training 3 times per week keeping movements slow and concentrated.
I hope this helped. Remember to be your clients biggest support and if you can try to get others in this clients circle of influence to support their choice in living a healthy life. These people can be make it or break it factors as well.