Excecute the following

sudo modprobe wire
sudo modprobe w1-gpio
sudo modprobe w1-therm

change the modules file by calling:

sudo nano /etc/modules 

and add the following lines at the end of the file:

wire
w1-gpio
w1-therm

if you use kernel 3.8 or higher then additionally execute:

sudo vi /boot/config.txt

and add the following lines

# Temperatursensor an 1-Wire
dtoverlay=w1-gpio
gpiopin=4

then trigger a reboot :

sudo reboot

to check whether it worked fine excute

sudo lsmod

which will give you some results similar to thins:

Module                  Size  Used by
i2c_dev                 6386  0
snd_bcm2835            22502  0
snd_pcm                92861  1 snd_bcm2835
snd_seq                58152  0
snd_seq_device          5142  1 snd_seq
snd_timer              22156  2 snd_pcm,snd_seq
snd                    67534  5 snd_bcm2835,snd_timer,snd_pcm,snd_seq,snd_seq_device
8192cu                556175  0
i2c_bcm2708             5988  0
w1_therm                4347  0
bcm2835_gpiomem         3703  2
bcm2835_rng             2207  0
w1_gpio                 4295  0
wire                   30987  2 w1_gpio,w1_therm
cn                      5756  1 wire
uio_pdrv_genirq         3526  0
uio                    10078  1 uio_pdrv_genirq

Important is that the three modules

wirte, w1_therm and w1_gpio are included in the lsit now...