I planned to develop wordpress themes commercially.. I just want to know the easy method to add plugins to my wordpress theme which I am developing.. can any one suggest me the simple steps.. and I need possible methods that I can use effectively..
You can always include the files of the plugins in your theme functions.php file. Of course you should put it into some reasonable structure to not get your theme bloated with the files and the code :).
You can use code like this (used in Carrington theme) to load all the plugins included in the folder "plugin" within your theme folder...
...then you use function
The simplest solution is just to use normal wordpress plugins ^^. Writing a specialized plugin system for your own theme is just totally unnecessary, makes your theme more bloated and increase development and maintaining cost. In this case the KISS principle wins.
Another point to consider when replacing plug-ins with custom methods would be updates. Many plug-in updates are to address security issues, or to resolve issues caused by WordPress Updates.