exercise at home

You can walk up and down stairs if you have some in your home. That's great cardio and will build up the strength in your legs.

Go for a run (or walk, whatever you can manage to start with).
Get some resistance bands, cheap (£10 a set) and you can do a ton of exercises with those.
Core exercises such as planks, mountain climber, etc.

I have a pull-up bar now, and some dumbells rather than the resistance bands. Between those and push-ups there are plenty I can do. I usually do pull-ups and push-ups after a run, and once a week do an upper-body workout.

My legs get a work out mountain biking where I make an effort to really pump the pedals rather than spin. You could do hill intervals or stair climbing.

I used to go to the gym but can't afford to now, and to be honest I'm enjoying my fitness routine more. Pounding the pavement and mountain biking are far better than being in a gym, and I can do the other exercises at a time to suit me without having to get my gym bag together, drive to the gym, pay £35 a month, etc.

You tube has a lot of great free videos...

Squats, lunges, push ups and planks will all help you build muscle without using equipment. And if you find that you want to work on your upper body more, you could always use water bottles, canned vegetables or a couple of books instead of weights.

^^This!

I go to the gym for weight-lifting but at home I do Leslie Sansone walking DVDs. I also hula hoop at home. Not with a fancy weighted hoop, just a $3 hula hoop I got at Walmart.