By default Fluxbox has four buttons on the titlebar, one on the left side (Stick) and three on the right side (Minimize, Maximize, Close)
You can change them by editing ~/.fluxbox/init Find the following lines:
session.titlebar.left: Stick
session.titlebar.right: Minimize Maximize Close
Then just change those. Possible buttons are (in Fluxbox 0.9.14):
MenuIcon
A button to show either the program's icon or default menu icon in theme
Pops up the window menu when clicked (same as right clicking the titlebar)
Stick
Toggles whether window sticks or not, meaning it is visible on all workspaces
Shade
Shades the window, only the titlebar will be left visible
Minimize
Self explanatory
Maximize
This too ...
Close
... and this
After editing, save the file and from fluxbox menu choose Config -> Reconfigure or Reload config or Restart'