We started with the cereal thing at 4 months...she was happy to try it. Honestly, she was much more "ready" at about 5.5 to 6 months. And, a lot of data suggests waiting until the 6 month mark for various reasons. (onset of diabetes in adulthood, weight gain etc...) I think it's up to you and what feels right.
We introduced a new veggie/fruit once a week just to make it easy. However, she got bored of the jarred stuff very fast and likes the real thing now! I can't blame her, I hate canned fruit/veggies
ANd, some of jarred combinations are downright gross. At 8.5 months we enjoy very thin slices of apple, mango, papaya, banana, strawberries and blueberries. (very messy) She loves Zwieback crackers, Baby Mumm Mumms, noodles, cooked rice, green beans, cooked carrots. We're having a blast watching her try new things. Enjoy!