You'll need to invest a fair bit of money if you want to go in on your own, for gear and qualifiactions. Most firms won't take part timers as the full time wages are shit enough as it is.
The law is getting tightened all the time so you need to seriously familiar with it's application to pest control.
What do you know about rats, mice and insects? Swot up because that'll be the bulk of any work you get.