Verified answer

Mark it with a pencil and ruler ;)))

The first thing you need to know whether the fabric is "on-grain" or not. (I'm assuming you're using a woven, not knitted fabric.)

If you're working with a woven-in plaid or widthwise stripe, it's easy. Just cut along the woven line(s). If your fabric isn't square, you can straighten it by pulling it from the corners in the opposite direction.

If it's a solid or a print, you could snip a bit at the edge, then tear it across. That would give you a straight line, but it's likely to stretch your fabric "off-grain", in which case you pull as above. The edges will be straight, but a bit fuzzy.

If you want a clean, straight edge,"pulling a thread" would be the way to go. Snip a spot along the edge, again. Then, instead of tearing the fabric, use a pin to pull one of the horizontal threads, so that it forms a straight, puckery line. (It'll be a lot clearer once you try it.) Then you can cut along that line. Again, if it's "off-grain", see above.

Sometimes, especially with some chemically-treated fabrics, it may not be possible to pull the fabric back on grain. If so, you have no alternative but doing the fold-and-cut the first Answerer suggested. If you do, the thing you are sewing will probably never hang quite right, but sometimes "A man's gotta do ...."

Have fun!

Depending on the type of fabric it may be really hard to do. First thing, if the fabric has lines or plaid, the lines in the fabric are not always straight on the fabric. The only way to get the center and get a straight edge is to open the fabric and lay it flat on a table or clean floor. Then measure from one corner to opposite corner. This should form an X. Find the center and mark it. Once you have found and marked the center, measure from the center to the end of the fabric. This should form a + on the fabric. At this point, keeping your markings, fold the fabric in half like it was when you bought it. Find the shortest point on the length from the center mark, then make a straight line across. This should be your straight edge. Good Luck!