In every i2c device data sheet you wil find a section detailing how to address your device in either write or read mode. the address is composed of 7 bits and most often most of these 7 bits are set in factory. How the remainig bits are set depends on how you designed your circuit.
let's imagine that you have a MCP3426 chip on your board, the data sheet is telling us in fig. 5.1 that bits 3,4,5 and 6 are set in the factory and bits 0,1 and 2 are set as shown on table 5.3 and therefore depend on how the two address pins are connected to the board (either earthed or connected to Vss or even left unconnected).
from this table and assuming your chip's address pins are earthed then your 7 bits address becomes: 1101000 (0x68)
now to read from the chip you have to shift those 7 bits by one bit to the right: which gives you 0xD0 and OR 0x1 (that is raise the read/write bit) which gives you 0xD1. that is the address you send when you want to write.
Should you want to write then the address becomes 0xD0 as the read bit is not raised.