I want to make it where sentences end at the same part of the line. like the bototm lines matches up to the bottom. like this
sdf sdf dfsfdsf.
sdf sfd sdfdsff.
they end matched up, i want to do this in my css external style sheet.
Do you mean that you want to justify the text? If so, you can use...
However, it is considered poor typography to use this on the web except in rare circumstances. This is because of a lack of hyphenation. The following link explains this in greater detail.
You might also be talking about the difference between monospaced and variably spaced typefaces. In a monospaced face like Courier or Monaco, lines with an equal number of characters will have the same length in pixels. But most typefaces are variably spaced to take into account the difference in width between characters. For example, "i" is much narrower than "m." Your example is using variably spaced type, but you have the exact same characters in both lines (just in a different order) so they add up to the same length.
If you are trying to lay out figures, like a math problem where they need to line up for comparison, try specifying a monospaced or "code" face in your CSS.
If you do in fact mean block justified type, see Jeremy's link!