experts suggestion required..need help for domain purchase

You should but blogname.com and you can have your blog at blogname.com or www.blogname.com. It doesn't matter much as to which one you choose.

If you want to monetize your blog:

1. Buy hosting + domain. (Wordpress hosting + domain) on bigrock.

2. Shift your blog to blogger. Buy a domain name. Buy the blog redirect option from wordpress so that your blogname.wordpress.com gets redirected to blogname.com

after buying wordpress hosting +domain name can i use them on my current blogname.wordpress.com site? do wordpress will charge for this? i mean still i have to pay any fees at wordpress?

i wordpress site it is given as,

Domain mapping is a paid upgrade that costs $13.00 per domain, per year. This fee is unrelated to the amount you pay your registrar for your domain itself. You will need to remember to continue renewing your domain with your existing registrar even after you have mapped it to WordPress.com.

that means yearly i have to pay fees right?

You are currently using wordpress.com which doesn't allow you to put adsense or any other ads no matter what upgrades you buy. 

So, you have two options

1. Shift to blogger

Costs Involved : Domain name ( ~10$ per year ) + Domain mapping upgrade on wordpress.com ( 13$ per month - you should buy this for atleast 1 year )

2. Shift to wordpress.org self hosted

Costs Involved : Domain name ( ~10$ per year ) + Domain mapping upgrade on wordpress.com ( 13$ per month - you should buy this for atleast 1 year ) + Wordpress Hosting (http://www.bigrock.in/web-hosting/wordpress-hosting.php)

Better, take a backup of your WP.com blog and migrate to a self hosted WP blog, where you'll be paying some bucks for your domain name and hosting fees. The amount might vary as per your choice of domain hosting, as most often, domain name charges are quite static though, increasing more frequently these days, ~700 bucks a year, for a .com domain, first year purchase is heavily discounted, where you can get the domain name for as low as Rs. 99. and GoDaddy offering a decent blogging level web hosting at as low as Rs. 99/month for the first year.

I won't suggest migrating to blogger as you'll find it very restrictive after using Wordpress, though, migrating to blogger will take care of hosting and domain costs (you can easily use custom domain name with a blogger blog, and they'll happily take care of hosting with almost no restrictions like no ads and such.)