To enable a pull up on a pin, you set the pin as a digital input and then set that pin's PORT bit high. For example:
DDRD &= ~(1 << PD7);
PORTD |= 1 << PD7;
This is just like connecting an external pull up to the PD7 pin. What it means is that the input is weakly pulled high, so this is what the pin will read if it is disconnected or connected to a tri-stated input. If you drive this pin low, it will easily overcome the weak pull-up.
If you have an input that is either low or floating (e.g. a push button that shorts your pin to ground when it is pushed), you should use the internal pull up so that you get a reliable reading while the input is floating. The pull up will have no meaningful effect on the pin if your input source has low output impedance.