Many babies are closer to six months before they are really ready to eat solid foods and trying sooner is fruitless. Other than watching for your baby's interest in food, your baby might start grabbing at your own plate during meals. If this is the case, it might be a good time to try out a first taste. Doctors will often say that starting with fruits is not a good idea and cereal is a better bet. What you try will depend on your own personal desires.
Bananas are a good soft food that easily mashes and can be tried by a baby with no teeth.
Using a spoon for a mashed banana will be the safest bet until you feel comfortable with the idea of your baby grabbing at a kernel of corn or a pea and trying something that is a little more solid and easier to handle. If it comes back up, your baby probably is not ready and waiting another week is a good idea. Once your baby is ready and not pushing foods back out, introducing one new food a week is generally safest in trying to prevent food allergies. Up to two items a week can be introduced depending on your family's history of allergies and what you feel most comfortable with. At first, starting new foods won't likely change any other feeding habits from the breast or bottle.