The association of pediatricians recommends waiting until 6 months of age to start solids if the baby is healthy & has no problems.
Generally, it's 1 new food per every 3 days, or longer (like 1 new food every 7 days) if you have a history of allergies (particularly food allergies). A food = 1 ingredient. So, apples = 1 food, an apple/pear combo baby food jar would equal 2 foods, and you should not feed it until after your baby has successfully eat the apples and the pears separately. (If he already ate apples and was fine, then you could feed the apple/pear combo & see if he reacted---that way, if he did react allergicaly, you'd know it was the pears. I hope that makes sense).
Juice shouldn't really be given for a while (we didn't give any until the baby was 1 year old, and then only sparingly/occasionally, and very watered down). Juice is basically flavored sugar water (that is how your body treats it), so it doesn't have much nutritional value and because babies like it so much it's easy for them to fill up on empty calories and that hurts their growth.
Most parents start with an infant cereal, with only 1 ingredient (like rice cereal), and mix it with formula or breastmilk. Our pediatrician actually recommended _not_ mixing it with breatmilk or formula, so that the baby would understand that drinks are for drinking (ie formula/breastmilk) and foods are for eating, and not get confused.
We opted to feed veggies first (we started with sweet potatoes & squash) and then did cereal then fruits. But I really don't think it matters much.
Keep in mind that for the first year of life, the baby should be getting all or most of it's nutrition from formula or breastmilk. The solid food is just an additional, small source of nutrition and you should not be relying on it to provide balanced nutrition until closer to 1 year old.
Definitely ask your pediatrician. They will probably have super easy handouts/guidelines to follow that will help guide you.