The starter on a BMW 318i has an important function: to start the car. It's also able to do so reliably several thousand times before it starts giving up for one reason or another. When it does, the only alternative is to replace it with a new model, otherwise there aren't many options left for you and driving the car that don't involve a manual transmission and a push start.
Pop the hood. Remove the air intake tubing from the engine using a flathead screwdriver, and set it to the side. This will expose the starter motor just below, mounted towards the firewall with several wires out of the top of it.
Remove the negative terminal on the battery then the positive terminal with an open-end wrench. Remove all connections at the starter as well, also using a wrench.
Unbolt the starter from the transmission using the 3/8-inch ratchet and socket. Lift it out of the engine compartment carefully, as it is quite heavy.
