Great question. Here's how:
Tap your avatar (player picture) on the bottom left of the main screen.
Note: Your buddy Pokemon won't show as walking with you. It will only show as an icon next to your avatar... *unless* you choose Pikachu. Pikachu will really walk with you.
The farther you walk, the more candy you can then earn for your buddy Pokemon. Depending on what Pokemon it is, you will earn one candy for every 1 to 5 kilometers.
Here are some examples:
1 Kilometer Buddies
Buddy Landel goes by Nature Boy.
you got 1 year buddy
To get spirit tomb in Pokemon diamond, you need to have an underground kit and a lot of time on your hands. What you have to accomplish underground is to meet 32 people. BUT, there is an easier way. Get a buddy to help you with this and have them stay underground. Get your buddy to come find you right after you go underground (don't move or when you go up you'll need to go find your buddy again). Keep going up and down, talking to your buddy every time you come down again. Repeat this 32 times. Then go to the weird rock thing right below the tower of ghosts. Put the ancient key in. A green blob should be on top of the rock. Don't worry, it won't disappear if you walk away. So go get some quick balls, go back to the rock and SAVE! Press A on the rock and right when the battle starts, use a quick ball, and voila!! You have a spirit tomb!
you need to go to a convention to get deoxys and stuff like that. or so I've herd.
Patrick Swayze went by Buddy.
There is no prize. Sorry, buddy.
Not possible buddy.
you go in the direct corner with a buddy and do it. youtube.com/iglowindadark
Buddy Valastro goes by Buddy.
Buddy Merrill goes by Buddy.
Buddy Cianci goes by Buddy.
Buddy Sosthand goes by Buddy Love.
Buddy Anderson went by Step Buddy.
Buddy Noonan went by Buddy Noonan.
Buddy Napier goes by Buddy Valentine.
the sailors buddy is in the bottom right room on the right side of where you come in
Buddy Roberts went by Buddy Jack Roberts.