I had a really great book called "Super Baby Food", I highly recommend getting it. Can't remember the author, but it is purple. It offers lots of advice on when to introduce certain foods. For example, it suggests avocados as one of the first food because the fat it contains is great for brain development. I bought organic avocados at Publix and still have an avocado tree I started from one of the seeds.
I started at 4 months, but my daughter really spit more out than she took in. It was really challenging, so I stopped and started again at 6 months.
Only give her one food at a time and do it for a few days to make sure she has no reaction to it. When she's ready to try a new food, introduce it earlier in the day so if she does have a reaction, it isn't at night while she is sleeping.
She probably doesn't need more than a tablespoon per meal in the beginning. If she eats rice and sweet potatoes, you can give her one at one meal and the other during the other meal.
Hope this helps. I remember the feeling of never knowing quite what to do. Try not to worry too much. At this point, eating solids is probably more of a learning experience for her than nutritional. My friend has a son that turned 1 in Feb and she just started feeding him solids in the last month or so. She nursed him exclusively for the first year and he is a very healthy 25-30 pound baby.