Hey, I would look to take it steady early on, and learn the ropes. Once you've got the hang of it, then you'll feel more comfortable taking on the larger offers (with higher profits).
I wouldn't recommend going for Profit Accumulator. I know that over on the Moneysavingexpert forums they are very critical of the site.
I've set up a website (matchedbettingadvisor.com) that sets out a beginner's guide to matched betting. It's completely free to use. I've written up some step by step walkthroughs, and some guidance on how to maximise your returns and pitfalls to avoid.
I'm trying to get the site off the ground, so take a look - I hope you find it useful!