Circuit training...

So im down 75 lbs and I want to firm up my belly arms and thighs...what should i be doing...joining a gym this week and just wondering what exercises I should focus on to build some muscle and help with losing these last 25 lbs...

One thing you might want to consider is a boot camp type class. They sound really scary (or at least they did to me). But basically they are just a bunch of circuit training activities. One think I liked about the boot camp idea vs a traditional gym was that I could find several options for them on groupon or living social and it let me try something out without the big expense or commitment of a gym. 

Exercise, while awesome for your overall health, won't do much for weight loss.  Focus on your diet and the last 25 pounds will come off.  

Check to see if your gym membership comes with a free training session - many do.  

I'm really a fan of p90x and 21 day fix.  Really any of the beach body dvd's are awesome.  They do a lot of circuit training with hand weights and resistance bands.  I've been doing them for about 2 months and my arms and legs have toned up quite a bit.  They work better for me than using the machines at the gym because it combines cardio and strength so it keeps your heart rate up while working all your muscles.

Think resistance training and weight lifting - much more effective for fat loss than hanging out on a treadmill for 30 minutes every day.  

Just remember - any time you start a new exercise program your body is going to rebel and hang onto tons of water.  So it will look like you're not losing (maybe even gaining) but you're not.  A gallon of water weighs 8 lbs...keep that in mind.