There are several configuration files that you can use to load functionality at startup. They have the form

($BaseDirectory | $UserBaseDirectory)/(Kernel | FrontEnd)/init.m

where $BaseDirectory is for every user on the system and $UserBaseDirectory is for you along and Kernel or FrontEnd specifies what you are configuring. In fact, a lot of the settings under the Preferences menu automatically write to $UserBaseDirectory/FrontEnd/init.m.

In your case, you are looking to add to $UserBaseDirctory/Kernel/init.m.

There are several configuration files that you can use to load functionality at startup. They have the form

($BaseDirectory | $UserBaseDirectory)/(Kernel | FrontEnd)/init.m

where

$BaseDirectory 

is for every user on the system and

$UserBaseDirectory

is for you along and Kernel or FrontEnd specifies what you are configuring. In fact, a lot of the settings under the Preferences menu automatically write to $UserBaseDirectory/FrontEnd/init.m.

In your case, you are looking to add to $UserBaseDirctory/Kernel/init.m.