Maybe you should be more specific of the type of strength you mean.
There's endurance-type strength which is what long distance athletes train for.
They use lighter weights but do many reps (between 15-25) and they tend to develop a more toned lean shape like Lance Armstrong.
This might work better for you seeing that you don't have access to a gym or heavy weights.
And then there's power-type strength that sprinters like Usain Bolt and powerlifters train for.
They use heavier weights and do less reps (between 3-8 ) but they tend to be a little more bulky (especially when you train more on the 8 rep range)
For power-type strength, maybe you could check out
Starting Strength by Mark Rippetoe which has an excellent strength training program for beginners
There's also
a thread in the bodybuilding.com forums that gives an outline of the program in the book just in case you can't afford to buy it.
The only thing is that you will need a barbell to do them
sad Maybe you could save up and at least get the bar alone? You can get good bargains if you look about on craig's list.
But both the book and the thread are still good reads because of the important principles they entail.
I don't mean to advert or anything, Gaia.
But in terms of bodyweight exercises for strength, here's something I came up with. It's something like circuit training.
Squats: 20 reps
Pushups: until just before failure
Pullups/chin ups: until just before failure
Situps/crunches: 20 reps
Seated Press: until just before failure
(use heavy books or something for the resistance)
Then take a 5-7 minute break (use it to stretch and for rehydration) and repeat the exercises in the order you see them.
By the end of the exercise day, you should have repeated all exercises 3 times.
You can probably do this 2-3 times a week but if you do, make sure you get enough rest. You can use the rest days for light-moderate cardio (active recovery)
Example-
Mon: exercise
Tue: rest
Wed: exercise
Thu: rest
Fri: exercise
Sat & Sun: rest