To render your TrueType font to a bitmap in the way that libtcod expects, you should use a separate library -- font rendering is a surprisingly complex task. FreeType is a very popular open source library for font rendering. You can find a Python interface here: http://code.google.com/p/freetype-py/. You will only need to use FreeType in a tool you'll use when developing your roguelike, not in your actual game.
First, determine what characters you will be using in your roguelike. Determine how to layout these characters on your font sheet. You can also simply choose to use the same layout as the one in the image you posted -- that's a sheet with 32 columns, starting at the space character (character 32).
Using your font rendering library, render each character by itself at the desired size. Pay attention to the size generated for each -- for instance, a '.' will be small and a 'w' will be large, even at the same font size. You must not just calculate a height, but a height above the baseline and a height below the baseline. (Example: if 'A' and 'g' are both 16 pixels tall, it's possible that you'll still need a rectangle taller than 16 pixels to align both correctly within it -- baseline-to-baseline.) Find the smallest rectangle size that will accommodate all of these sizes -- this is how large each cell in your font sheet must be.
Once you know how large your sheet will be, make another pass through all the desired letters to construct your bitmap, putting each letter in its cell. As far as y-positioning goes, all baselines must be aligned. If some characters are wider than others, you can choose to center or to left-align each character within its cell. (Each of these will come with its own weirdnesses -- you're really going to want a fixed-width font for a roguelike.)
Additional tips:
- Use antialiasing. This'll make your font easier on the eyes than pure
monochrome.
- Don't use colour, render your font in grayscale. libtcod has
functionality to generate coloured text from your grayscale
fontsheet.
- Consider whether you want a square font or not. The advantage of a
square font is that "circles" in your roguelike world will look like
circles on the screen. The disadvantage is that square fonts are
generally "uglier" and harder to read.
