I started matched betting about 3.5 years ago but had a year out so been active for 2.5 years. I read about it and watched videos for months before taking the plunge. I started with £100 as I was happy I could afford to lose it if it all went wrong. I've only ever used my winnings to fund new offers, never put a penny more in to it. I've made about £14K, 5K from matched betting and 9K from casino.
After the initial few months which were very slow, whilst building my funds and learning how to do things I was making £2-300 a month on matched betting. Started to get gubbed (banned from taking part in their offers) by a few bookies, so looked in to casino. I am now gubbed from the vast majority of sports bookies so most of my income is from casino offers and make on aver £5-600 a month from 10-15 hours a week.
Matched betting is pretty much guaranteed profit as long as you follow the guides. Start with this to build up your bank roll and your confidence.
Casino is more risky, you will win some, you will lost some, but on average you will profit. The last few months I have either lost or broken even but this month looks like I've made £1K ish, on average over 18 months its £5/600 a month.
I'd recommend Team Profit and Odds Monkey for guides and tutorials. Watch the videos, read the instructions, think you understand, ask questions on the facebook newbies page, read the instructions again, watch the videos again, do exactly as they say and go for it. Before you begin, set up a new email, specifically for matched betting, and open a new bank account so that you can keep everything separate from your day to day banking/life. I just opened a new current account with my usual bank (Halifax) and never had any issues.
Good luck