Developing a plugin or theme is not as difficult as uploading this to WordPress server. right? at least it was true for me in starting.
After developing a plugin i uploaded it into wordpress database after 1 month. But when i started to update it, i took only some minutes to understand the complete process.

Let’s start

Develop your plugin and test it locally.

Notify WordPress team about your work

a. Create you account on wordpress .
b. Select “Add your plugin” from left menu. Fil basic information about your plugin or theme. So that wordpress team can access it and test it.

Wait for mail
Once your plugin is submitted to and tested by wordpress team you will get a mail. like this

    Your plugin hosting request has been approved.
    Within one hour, you will have access to your SVN repository at 

http://plugins.svn.wordpress.org/YOUR-PLUGIN-NAME/

    with your WordPress.org/bbPress.org username and password (the same one you use on the forums).
    Here's some handy links to help you get started.
    Using Subversion with the WordPress Plugins Directory 

http://wordpress.org/extend/plugins/about/svn/

    FAQ about the WordPress Plugins Directory 

http://wordpress.org/extend/plugins/about/faq/

    WordPress Plugins Directory readme.txt standard 

http://wordpress.org/extend/plugins/about/readme.txt

    readme.txt validator: 

http://wordpress.org/extend/plugins/about/validator/

    Enjoy!

It may confuse you little bit. But forget it

4. Download “Tortoise SVN” from http://subversion.tigris.org/. [you may have to login to download it]

5. Once the installation is done.

1. Create a folder somewhere on your hard disk.
2. Right click on the folder. And select “Create repository here”.
3. Some folders will be created inside the folder you opted for repository.
4. Again right click on the same folder. And select “SVN Checkout”. [To know more in detail you may visit wordpress]
5. It will ask for login ID & Password. Use the id you use to login on wordpress plugin site.

6. Inside your plugin directory you will get three more folders

1. trunk
2. tags
3. branches

7. Copy your plugin files in “trunk” folder.(dont archive them)

8. Make a readme.txt to tell something about your plugin or theme. refer its format.

Note
To check whether contents of your readme.txt are valid or not. Go to read me validator. Here you can paste contents of readme.txt. And can validate it.

Extra files
Providing screenshots to viewers of your plugin is really a good idea. It may fascinate users to use your plugin. Take some screenshot. And place them into “tags” folder of your repository folder. And specify their path into readme.txt.

Note
tags folder contain a version folder like “/2.0/”. All files are placed only inside version folder. This version must be similar to the version specified in you readme.txt

Save your work

What else. Just right click on your repository folder and select “SVN Commit”. Your work will be uploaded to wordpress server. You may check it by following http://plugins.svn.wordpress.org/YOUR-PLUGIN-NAME/

You may also refer my repository directory structure for further help.