You can activate VIP by using VIP activation items. If you don't have any, you can purchase them using Diamonds.
Note that if you use multiple VIP items, the duration will stack.

VIP grants you extra bonuses that vary depending on your VIP level, along with a daily free reward.
It also gives you access to the VIP Shop, where you can purchase exclusive items.
Some items require a certain VIP level to be unlocked, and the shop's contents refresh daily.

To access the VIP Shop, tap the VIP icon next to your avatar in the top-left corner of the screen, then select VIP Shop.