S.D.
I started giving my baby food in a feeder at about 5 months. He was ready because as you said, he was sitting up and he was showing interest in what we were eating. I started with rice cereal. The doctor's now recommend you DO NOT give it to them in a bottle, feed it to them with a spoon, so they get use to solids on a spoon. After a month or so, try to add fresh mashed up fruit in a feeder or baby food. The feeder looks like this:
http://www.babysafefeeder.com/press10.htm
By the way, look into the foods you should NOT give a baby until after their 1st birthday. Egg whites, strawberries, honey, etc.