When I started to develop plugins for Notepad++ the following links have been helpful:
How to develop a plugin or a lexer
Messages and notifications
Sources of plugin interface
Regarding toolbar buttons
The handling of toolbar buttons is totally transparent to a plugin developer because they work like a menu entry.
A plugin developer has to fill an array at plugin’s startup that contains structs of type FuncItem. Every array element describes a menu entry of the plugin (text, click handler function, menu command id, Checked state, and keyboard shortcut).
Notepad++ queries a pointer to this array at a certain time in the startup process. It generates the menu entry of the plugin in the Plugins menu, adds for every array element a submenu entry and fills the menu command id member of the array elements. So you know the menu command ids of your menu entries.
If you want to have a toolbar button you have to hook the NPPN_TB_MODIFICATION event sent by Notepad++ to your plugin. In the handler of this event you have to fill a struct of type toolbarIcons (please note: you have to fill only one of its two members, either hToolbarBmp or hToolbarIcon but it seems only the first one works). The bitmap whose handle you provide has to be in a 8 bits-per-pixel palletized format (max. 256 colors) and its size has to be 16x16 pixels. Then you can send a NPPM_ADDTOOLBARICON event to Notepad++ where you provide the menu command id of the menu entry which you want to connect with your toolbar button and a pointer to the toolbarIcons struct you filled before.