Several horizontal and vertical lines, also known as glyph metrics, will help you with your font design. You can activate them through the Metrics Options window.
In order to change the horizontal metrics click Settings on the Format menu and then click the Metrics tab. WinAscent, WinDescent, TypoAscender and TypoDescender are located on the Metrics tab while CapHeight and x-Height can be found on the Ranges tab.
In the Glyph Edit window there are two vertical lines (normally the glyph outline lies in between these lines) that represent the left and right side bearings. These are shown by default but you can hide them through the Show Metrics button on the Drawing toolbar. The left and right side bearings can be changed by dragging them to their desired position. You can also adjust the bearings when you right click a glyph in the Glyph Overview window and select Properties. Here you can alter the left side bearing and the advance width. White space should be evenly distributed between the left and right side bearings of glyphs except when font is specifically designed with ligatures for joining characters in a cursive script font.
Note: Lining numbers (i.e. the digits 0 - 9) should all have the same advance width.