What is the best way to start investing on a fixed income?

My wife and I are both on Social Security.We dont have alot of money,but enough to invest $50 to $100 each month.We are in our early 40's and would like to have a little nest for the future.I am very green when it comes to investing.I am a little leary about investment firms. I have heard some bad things.Dont know which ones i can trust.I would like a deversified portfolio just dont know how to safely deversify it.Also could you recomend a good book to get me started in understanding the investing world a little better.(stocks,bonds,mutuial funds,401's,Ira's)ect...A little advice would be greatly apriciated. THANK YOU TRENT& GINA. SAC, CAL.