How to get an agent
First & most importantly is - DO YOUR RESEARCH!
A quick google will bring up lots of search results but not all of them will be legit and / or running in a way that is in your or your childs best interests.
The best way of finding out about an agency/agent you are interested in or are interested in you is through recommendations & others experiences.
Websites such as this, The Stage, NETMUMS & notapushymum.com, plus others are a great source of information for like minded performers and parents of child performers who will be able to advise you on who to avoid & who to get in touch with. I used NETMUMS when my daughter was first starting out as there were plenty of opinions on there and the mums (and dads!) definitely knew which agency's were scammers!
Second is to apply to all these legit agency's, some may come back to you quicker than others so don't be hasty in your decision if you are hoping I hear on more news. Most agents now expect to meet with you or your child, so be prepared to have to go along to a few workshop or individual auditions, which may only take place once a month or less often. When you have your shortlist of 'yes's' go with your gut instinct as well as recommendations. All the research you have one will hopefully give you peace of mind that you are not being scammed.
Last - remember that even having an agent isn't always going to guarantee work. If an agent says that they will definitely get in or your child work then they are telling porkie pies! No agent will know what work is coming in from one day to the next. What might be a popular look or skill one week will not necessarily be so the next.