I want to create my own form that are submitted and values of that form will be stored in joomla database. How can i used the Joomla connection code to interacte with the joomla database.
Answer
If you just want to get your work done, there are some components already avaiable in Joomla Extension's website to your form's needs. CkForms is a good one, you can create simple forms with it that are automatically saved in the database (supports upload of files too).
If you want to learn how to write extensions in Joomla, you should read Joomla Official Documentation website.
Extensions in Joomla are divided in "Components", "Modules" and "Plugins". More informations about these differences can be found here and here.
What you're trying to achieve sounds a "component" to me. Downloading the CkForms and reading it's source code should get you started to Joomla's way of writing components.
EDIT: Joomla has a huge API with a lot of features. It has a database module of it's own, with insert methods and such. Reading Joomla's API website before implementing your component it's a good idea to avoid "reinventing the wheel" and it's a good practice since those methods are extensively tested by all Joomla users.