For most of them, you have to use a HM.:)
Tailow requires: National Poke`dex, Rock Climb & Surf
Go to Cherrygrove City and use Surf left of the town .You will see land where you use Rock Climb. Use it. Use Head Butt up on the lower left tree.
Slackoth requires:national Poke`dex_ Go to route 25. Go left. You will come to 4 trees. Use Head Butt on the 2nd to the left tree.
Starly requires:national Poke`dex_ The tree Starly is located in is in Pewter city. However, it is not accessible within the city itself. In order to get it, you have to get to the small fenced area on the left side of the city entrance.
Burmy requires:National Poke`dex & Rock Climb__You know that one farm? Go down from there until you find a place where you use Rock Climb. Use Rock Climb .Follow the path. Once you get to the end, face down and use Head Butt.
Cherubi requires:national Poke`dex & Rock Climb_Go to National Park. Find a place where you use Rock Climb. Use Rock Climb. You will be on a ledge .Use Head Butt on the top tree.