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: Sticksession.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'