Hello there. To answer your questions:
1) Typically the AGP slot is brown in color, but not in all cases. Gigabyte boards for instance have AGP slots that are day-glo green color. So color doesn't tell you much in some cases.
2) The AGP slot is positioned closer to the center of the board, farther away from the edge then the PCI slots (beige or white in color typically). The AGP slot is slightly shorter in length than the PCI slot, and positioned about in inch farther towards the center of the board from the edge than the PCI slot. You know you have onboard video (video thats integrated into the board) by looking at your motherboard along the side where the ports are. It will be blue in color, and have 15 female pins (or holes rather--3 rows with 5 holes each).
3) Onboard video actually shares your system's RAM--this is usually referred to as the AGP aperture. Suppose you have 256 megs of system memory for example, and you set the AGP aperture to 32 megs. This will make your available system memory to be 224 megs, because you just reserved 32 megs for video memory. Onboard video cannot really be upgraded--the best you can do is to keep it updated using the latest video driver for your motherboard, or purchase a video card that will be compatible with your motherboard.
I hope this answers your questions.