Assuming you have got microsoft paint (it should be loaded as standard with windows), run the program and do the following:
First open your picture - unfortunately you can only zoom out to 100% so will have to scroll the screen to where you want the text.
Go to the image menu and make sure the item "Draw Opaque" is
NOT ticked
Go to the view menu and make sure "Text Toolbar"
IS ticked.
Now, select the text tool from the toolbox on the left - it's the 5th one down on the right column that looks like a capital 'A'
You're now going to draw a box to hold the text.. click where you want the top left corner to be, then, holding down the left mouse key, drag the box out to the size you want. Then let go the key to leave a dotted box.
Now type the text you want.. don't worry about the size or colour yet.
To change the colour of the text, click on one of the colours in the pallet at the bottom left of the screen..
To change the font, go to the Fonts toolbar that should now have appeared - it will have 3 white boxes which are the font type, size, and character set. Play around with the font and size until you get what you want - don't worry about the character set, leave it set to Western.
To the right of these boxes are buttons for Bold, Italic, Underlined, and the last one i have no idea but it's disabled. click them on and off to see the effect.
When you're happy, save the file - i'd tend to save it under a new name to keep the original safe.
a couple of things to note:
1, view in normal 100% size - The text tool only works in 100% view mode
2, Once you have positioned the text box it is possible to move it by dragging the corners, but it ain't easy!
3, If you click outside the text box, it will disappear and the text will be stuck where it is - use Edit->Undo(Ctrl-Z) to get rid of it and try again
let us know if you try it