If straight lines in an original appear crooked in a scanned image, make sure to place your original straight when you scan it.
Clean the scanner glass with a soft, dry, lint-free cloth or use a small amount of glass cleaner on the cloth, if necessary. Paper towels are not recommended.
If you are scanning from the ADF, make sure the ADF edge guides fit against the loaded originals. You can also try cleaning the ADF and scanning again.