If you have a community or junior college in your area, chances are it offers bookkeeping classes and increasingly these classes are being offered online. Bookkeeping classes can be offered for credit or as non-credit continuing education. Check your school's website to see what is offered.
Professional associations such as the AIPB (American Institute of Bookkeepers) and the NACPB (National Association of Certified Public Bookkeepers) offer online bookkeeping classes. Check their websites at aipb.org and nacpb.org for more information.
Makers of bookkeeping software like Quickbooks often offer online training. Quickbooks classes can be found at Intuit.com.
Once you've checked out the different options for bookkeeping classes, choose one, taking into account the type of training and the cost involved. Many online classes are self-paced so you may be able to begin the class right away. If not, make sure the schedule works with yours.
Finally, obtain any books or other supplies you need for the class. Take your commitment seriously, complete all requirements, and soon you'll be trained as a bookkeeper.