The headstock is where most of the string changing action takes place.
On the headstock there are tuning posts around which you will wrap the strings.
Each tuning post has a little string hole in the center. As you will see, the end of the string is placed in the string hole.
A tuning key is connected to each tuning post. Turning the key one way tightens the string (making the pitch go higher); turning the key the other way loosens the string (making the pitch go lower). The direction depends on your bass and how it is strung. You need to listen for which way is which.
On some headstock designs string trees help hold the strings at the proper height. Some string trees look more tree-like than the one pictured above. They often look like a disc or button.
The nut is the part located between the headstock and the fretboard. The nut holds the strings at the proper height over the fretboard and keeps the strings spaced correctly.
There are a couple of common headstock designs. There is the straight headstock found on Fenders, Fender knock-offs and others. These headstocks go straight in line with the neck.
Then there is the angleback headstock found on most modern designs. The angleback headstock angles away from the straight line of the neck. The angle of the headstock helps pull the strings tightly over the nut. On straight headstocks the string tree helps pull the strings tightly over the nut.
The bridge is the metal plate that is bolted to the body of your bass.
For each string there is a saddle on which the string rides (yee-haw!). The string saddles hold the strings at the proper height and spacing on the other end of your bass. They are adjustable with screwdrivers and hex keys, but I don’t recommend adjusting them unless you know what you are doing. There’s a lot more to adjusting the height of the strings than you may realize at this point.