Parents used to be able to assist children with building a good credit score by adding them as an authorized user on a card. In the past when you were an authorized user on your parent's cards you were able to borrow their good credit rating as well. When giving someone a rating FICO would essentially attach the credit records of people who listed you as an authorized user to your credit history. That way a child who wanted a card could first be added as an authorized user on their parent's card and then when the child applied for his or her own credit card, the parent's credit history was used to give that child a good credit score even though that child had no individual credit history.
Well that can't happen anymore. FICO closed that route about a year ago because they found people were actually making money by adding others to their credit cards as authorized users (for a fee, of course) to help people with bad credit ratings get good ones. You may still get a scam email promising to fix your credit score. If someone says they can fix your credit score quickly for a fee, run, don't walk away.
Unfortunately, if you do have a bad credit score there is no quick fix unless that score is based on inaccurate information. For example, if you look at a copy of your credit report and see credit items that are not yours or that do not accurately reflect your payment history, correcting those errors and getting them off your report can improve your score quickly. But, if the information on the report is accurate, the road to improving that score can take a long time.