Strength training at home??

resistance bands for a cheap start?

pushups, pullups, squats, lunges would be a good start.

You need to challenge yourself to force your body to change though so at some stage, you will need to add resistance. You can do this with variations of the above e.g. squats to one leg squats, pushups to one arm pushups. I would check youtube for various exercise variations of the above

Brain Over Brawn has a nice write-up on home strength training. http://brainoverbrawn.com/get-the-book/ (Free)
In summary, get a heavy duty duffel bag. Fill it with heavy things. Do deadlifts, squats, and overhead press with it.

You can load up a backpack and wear it to do push-ups and squats.

Once you can complete 2 sets of 15 push-ups, start elevating your feet for them, on a chair, stool, or even a stack of books. Work your way back to completing 2 sets of 15, then elevate your feet higher. After you feel like even elevated push-ups are way too easy, then you can put your knees on the floor and start working one-arm push-ups, or wear a backpack for weighted push-ups.

Do squats! Even if you are doing them with just your body weight. Make sure you squat nice and low. Your hamstrings shouldn't come to rest on your calves, but you want to get deep with them. Keep your weight on your heels. If your heels come off the ground, then your weight is on your toes, which is poor form.
If these get too easy, either hold something heavy while you do them ("goblet squats"), or try pistol squats (one-legged squats). Feel free to hold onto a door frame for balance if you try these.

Try burpees: http://exrx.net/Aerobic/Exercises/BurpeeAdvanced.html. You can make these very intense by doing them for speed, and with a big jump (or, if you're strong enough, do clap push-ups with them).

Simple tip: find something heavy sitting around. Bend down and pick it up, then lift it up over your head. Repeat until strong. Keep your core muscles tense and your back straight and erect.

Try planks for your ab workout, as they do not require repeated spinal flexion and thus might be easier on your lower back: http://exercise.about.com/od/abs/ss/abexercises_10.htm
Remember that spot reduction is a myth. In other words, you will not burn fat from your belly by doing ab workouts. Your body decides where the fat comes off, and you can't change that without getting lypo or something. "Cutting gels" and related products just sap water from the area, and this effect is temporary. I very seldom do ab workouts, as they get all the work they need through supporting my spine during lifts, and from rock climbing. Overdoing ab workouts can make your stomach look boxy and masculine, which I doubt you want.

Get a door frame pull-up bar. This is one of the best upper-back exercises. If you can't do a pull-up yet, then either assist by having a foot on a sturdy chair, or do pull-up negatives (jump to the top, lower yourself as slowly as possible). Eventually you'll be able to do regular ol' pull-ups and will have some nice upper back definition to show for it.

Do interval training on the bike. Instead of riding for 30 or 40 minutes at a steady pace, try a shorter workout of alternating steady and sprints. Warm up for a few minutes, then pedal VERY FAST for 20 seconds. Then do a regular pace for a minute, then SPRINT for another 20 seconds, etc. 10 minutes of this is enough for a very good workout, as long as you're not cheating yourself on the sprint—go HARD.

Ignore anybody who tells you that you'll get "big and bulky" if you strength train. Unless you are a genetic freak (i.e. a woman with the hormones of a 16yo male) or taking growth hormones/steroids, you will not gain a ton of muscle mass. Even if you could, it would take years of dedicated work and LOTS of food. Nobody has ever accidentally become a bodybuilder. Strength training is a key part of "toning," as you need to maintain muscle mass in order to have muscle tone once your bodyfat % reduces.

Feel free to message me if you have any other questions. Good luck!

I rarely use them, but I bought most of my weights from thrift stores. You can get them pretty cheap that way. I use them on the days I can't get to the gym, but still want to workout.