To trim your project back down to size, count the stitches in the last row to determine how many extra stitches you have. If you have more stitches than you're supposed to, carefully pull out the last row and count the number of stitches in the next row. Continue pulling rows out one by one, counting as you go, until you're back to the correct number of stitches. Then you can start over safely.
What to do if you've added only one or two stitches? Just subtract the same number of stitches in the next row by working two stitches together until you're back to the correct stitch count. After you've decreased the stitches, hold up your work to make sure the decrease isn't noticeable.
To keep from having to redo your work in the future, make sure to count your stitches after every row until you feel more confident in your crochet skills. And don't forget that turning chains count as the first stitch in taller stitches; make sure to work into them at the end of a row.