Okay, drawing can be a little confusing. Each 8x8 tile is 16 bytes(2-bit color encoding). Okay, first you need to get a tile number from the Tile Map Table(0x9800 or 0x9C00). You then use that tile number as an offset from the tile data table at either 0x8000 or 0x8800(0x8000 is 0 to 255, 0x8800 is -127 to 128). Make the tile number unsigned or signed depending on the Tile Data Table location, then multiply the number by 16 and add it to the Tile Data Table address(example: tile number is 240. 0x8000 + (240 * 16) = 0x8F00). Each line of the tile is made up of 2 bytes. You take the bits in the first byte and add them with the bits in the second byte. If a bit in the first byte is on, it is 1. If a bit is on in the second byte is on, it is 2. So, if bit 0 of byte 1 is on, and bit 0 of byte 2 is off, the value is 1. If bit 0 of byte 1 is off, and bit 0 of byte 2 is on, the value is 2. If both are on, it's 3. This value is used with the BGP I/O register. If it 3, the color is gotten from bits 6-7. If it is 2, bits 4-5. 1 is bits 2-3. 0 is bits 0-1. The value you get(0-3) determines the color to draw. 0 is white, 1 is light grey, 2 is dark grey, and 3 is black. Hope that was clear

That should be everything you need to know starting out.