Ah, well, that's not so easy
I'll go through the steps, and you'll have to work out the details:
1. Pick a non-proportional font for simplicity. One that fits nicely in 16x16 squares for example.
2. Render this font ahead of time into a 256x256 luminance alpha texture (ie. monochrome with alpha channel), so that each character in the font occupies a 16x16 tile. Each character could be positioned according to its ascii value; eg. 'A' is 65, so it would be 1 across and 4 down at (16,64).
3. When the time comes to render strings in OpenGL, bind to that texture, and render quads according to the characters in the string you want to render. Texture coordinates are in fractions of course, so instead of 0..256 coordinates you're dealing with 0.0f to 1.0f, so scale them accordingly.
That's the simplest way to do it.