There are scales which will do BMI and body fat for that king of thing, I believe you can pick them up from about £15 obviously but you'll want to spend around £20 upwards depending on make etc for something accurate

As for getting your abs/muscle, just make sure you are taking on enough protein to make the gain. I found to make gain I had to consume about 3000 cals a day which had as much protein as I could squeeze in with with complex carbs and very little fat and sugary carbs so that I could put on muscle.

Doing it properly you should be able to make about 1st of difference in 4 weeks, but that's as a total so you might add 8lbs muscle and lose 6lbs of fat...