A quick tip in case you run into the following problem as I did with a Raspberry Pi 3 running the Jessie version of Raspbian.

Problem

Unable to access the Internet using the Raspberry Pi’s built in WiFi.

Symptoms

1. You can successfully connect to the WiFi network
2. You are unable to access any Internet sites.
3. Running the command route -n in a terminal shows a default gateway of 0.0.0.0
4. Within the file /etc/network/interfaces you see the following:
   …
   iface wlan0 inet manual
   wpa-conf /etc/wpa_supplicant/wpa_supplican.conf
   …

Resolution

Using your favourite text open the file /etc/network/interfaces and find the following line:
…
iface wlan0 inet manual
wpa-conf /etc/wpa_supplicant/wpa_supplican.conf
…

Amend or comment out the line iface wlan0 inet manual and change it to:
…
iface wlan0 inet dhcp
wpa-conf /etc/wpa_supplicant/wpa_supplican.conf
…

The only change is shown in bold i.e manual has changed to dhcp. The screen shot below shows how the interfaces file looks with the original manual entry commented out and the corrected line with dhcp added:

Reboot your Raspberry Pi, run the command route -n and you should see a new line with the
gateway details once confirmed open your Browser and you should now be able to connect to the Internet.

What is DHCP?

The following is taken from here.

Dynamic Host Configuration Protocol (DHCP) is a network protocol that enables a server to automatically assign an IP address to a computer….