ahhh. . .the transition to solids is a hard time.
First I am hoping you are starting the solids under the instruction of your pediatrician. Solid foods are introduced between the age of 4-6 months depending on who you ask.
Second, one food group at a time. Most doctors ask you to start with soupy rice cereal, diluted in breast milk or formula.
Then progressing slowly to the orange colored food group- squash, sweet potatoes, and such. Doctors ask that you introduce one color food group a week. If your baby tolerates you can progress to other food groups.
This will help you detect any allergies or reactions, which will be harder to determine if you give fruits, rice, squash and green beans all in one week.
The first link here should be the most helpful, it is a chart that is for birth-24 months on what to feed and when.
At this age you pediatrician should be giving you printed information on every visit that has direction and suggestions on what to do between visits and what milestones and feeding advances should be made.
This site (see the first link below) also gives a great list of choking hazards and such. I think it could be a really great resource for first time moms that have a lot of questions.
Here are some helpful sites:
http://www.askdrsears.com/html/3/T030500.asp
http://www.keepkidshealthy.com/infant/startingsolids.html
http://www.babycenter.com/refcap/baby/babyfeeding/113.html
http://www.askdrsears.com/html/3/T032000.asp
As far as portion size. It depends on your child. My son was a big eater because he is a big boy so sometimes he surprised me by how much more he wanted to eat than I thought he would be able to take. I asked other moms and my pediatrician what was normal. I got the same answer from everyone.
Babies don't over eat. I was told by countless moms and medical staff, children will stop eating when they are full.
With babies this means spitting out the nipple of the bottle or breast. With older babies and toddlers this means turning their head away when food is offered or spitting out what you just spooned in. With older toddlers this means throwing the food on the floor or starting to really play with it.
But look at your own childs cue. Your baby might be fussy from gas if he is actually full. When my son transitioned to solids he was much harder to burp.
Try patting him on the back more vigorously to get some burps, or lay him on his stomach across your legs and pat him firmly, or lay him on a soft but solid surface on his stomach (face turned to the side) and pat his back firmly, or lay him on his back- hold one foot in each hand and push his knees toward his stomach. Or you can lay him on his back and massage his stomach area gently.
All these tricks get the gas out and may make him less fussy.
Good luck, and you are not clueless, just new to this and there is no instruction manual.