Permalinks not working

Hey everyone,

I have hosted wordpress on my site…..
I have PHP 5 and MySQL5 on my hosting…..(I dont know which version am i using for WordPress)

The default config for permalinks still works….

but when i change…..

IT DOESN’T……
and i cant even access my home page…..

when i try to access my site : https://arjunbajaj.com it doesn’t load and shows me error 404, page not found…

but when i type https://arjunbajaj.com/index.php it loads………

and there’s also another problem…….
when i change to the already available permalinks in my wordpress 2.9.2, some of the posts and pages load as single pages…….

but when i put my own permalinks the posts and pages even don’t load….

i guess it is some .htaccess problem……

but i really don’t know what it is and actually i’m not a wordpress master so i really dont know anything except using the UI some FTP and some code but not much of html and php……cause rite now i’m learning java…..

i have to change my permalinks to only the names i specify, like arjunbajaj.com/<post name which i can change>

what should i do???

and i really need to get that to work fast or my site will be in a mess cause i have to work with post urls……

please help……

thnx…..