I've always maintained that everyone can sing. I've never encountered anyone who was truly "tone deaf".
Ask somebody a question. Say, "Would you like a cup of tea?" Notice how the pitch of your voice changes? Perhaps it goes up on "tea"; perhaps for that more sympathetic tone it goes down at the end...
The fact is, you're altering the pitch and tone of your voice at will. Higher and lower; louder and softer; longer and shorter... Singing is exactly the same, though of course it's a little more regulated.
So if you think you can't sing, try this:
• Make the highest sound you possibly can
• Make the lowest sound you possibly can
• Make a sound somewhere in the middle
Now you've made three notes of different pitches.
See if you can make a sound in between the middle one and the low one. That's four.
Now a sound between the middle one and the high one. That's five.
That's where we all start. With practice - and, of course, time - you learn how to divide these gaps into ever smaller "slices" and how to reproduce each sound at will.
So sing often, sing without shame, listen to the sounds you're making, and enjoy the sounds you're making!
And if you're somewere in the London area, maybe you might want to check this out - a choir for people who "can't" (for want of a better word) sing!
Now go and make some noise!