I started mixing a little cereal with breast milk at four months. I didn't start anything other than cereal and BM untill 6 months. All of the steps are for progression from easily digested foods to gradually increased not so easily digested foods. The rice and oatmeal cereal are first because they are easiest on the very young system to digest. Then there are other cereals like barley, wheat, and mixes. Some cereals also include banana, which both my kids enjoyed.
After six months you can move to jar foods or your own unseasoned purreed foods. That is when vegies and fruits can be started. Starting vegies before fruits increases the chances of the child liking and accepting vegies. Both my kids love veggies and fruits, so that method seemed to work well.
No matter what you give, make sure that you give each new food for around a week or two consecutively to make sure no allergic reactions go unnoticed. The meets are the last foods to start the baby on because they are the hardest to digest. I think around a year or 2 is a sufficient wait to start meats.
There are also big allergy producers that should not be given until they around 2 or 3 years. I don't remember exact ages but they are products containing peanuts, honey, shellfish, and things like that.
Anyway, have fun whenever you do start solids. They can be quite a fun mess.
V.