user47841: I don't know any thing about bit coin , what I know that it digital currency maintained by people and not banks.
How do I start earning my bitcoins ? OK After earning this bitcoins ? What all things I can buy in real world or it just to pay for only services to service provider?

user44734: HOW TO BUY BITCOINS
1) From exchanges. Exchanges are many based on countries. UNOCOIN is a popular one for India. Alternatively, if your nationality is different from what I presumed, you can have a look at this site for many exchanges and wallets: COINDESK. You can create a bitcoin wallet here: BLOCKCHAIN
2) Have a look at this link for more detailed explanation: How do you obtain bitcoins?
WHERE CAN I SPEND THEM
You can spend them on websites/services which ACCEPT bitcoins as a mode of payment just like Paypal, Credit Card or Debit Card. 

user48073: As a supplement to the buying process provided by @trollster -
If you would like to earn bitcoin (rather than paying for them directly) there are a few ways to do so.  Mining is one example, but it requires purchasing specialized equipment and will likely entail a good deal of research as well.  You might choose to sell goods or services in exchange for bitcoin.
A more simple and practical way to earn small amounts of bitcoin is by completing tasks or surveys online.  There is an array of websites that offer this form of compensation - which will vary in terms of compensation level, accessibility, reputation, and reliability.  From my own experience, I can attest to the fact that www.BitcoinGet.com did pay out (which was my biggest concern).  Earning a full bitcoin through this medium alone would take quite a bit of time - but is the only option I can recommend that requires zero capital.
If you do want to earn some bitcoin in that manner, you should still create a bitcoin wallet and an address by following step 1 from @trollster
Welcome to Bitcoin!

47841: <p>I don't know any thing about bit coin , what I know that it digital currency maintained by people and not banks.</p>

<p>How do I start earning my bitcoins ? OK After earning this bitcoins ? What all things I can buy in real world or it just to pay for only services to service provider?</p>
 44734: <p><strong>HOW TO BUY BITCOINS</strong></p>

<p>1) From exchanges. Exchanges are many based on countries. <a href="https://www.unocoin.com" rel="nofollow noreferrer">UNOCOIN</a> is a popular one for India. Alternatively, if your nationality is different from what I presumed, you can have a look at this site for many exchanges and wallets: <a href="http://www.coindesk.com/information/how-can-i-buy-bitcoins/" rel="nofollow noreferrer">COINDESK</a>. You can create a bitcoin wallet here: <a href="https://blockchain.info/" rel="nofollow noreferrer">BLOCKCHAIN</a></p>

<p>2) Have a look at this link for more detailed explanation: <a href="https://bitcoin.stackexchange.com/questions/91/how-do-you-obtain-bitcoins">How do you obtain bitcoins?</a></p>

<p><strong>WHERE CAN I SPEND THEM</strong></p>

<p>You can spend them on websites/services which ACCEPT bitcoins as a mode of payment just like Paypal, Credit Card or Debit Card. </p>
 48073: <p>As a supplement to the buying process provided by @trollster -</p>

<p>If you would like to <em>earn</em> bitcoin (rather than paying for them directly) there are a few ways to do so.  Mining is one example, but it requires purchasing specialized equipment and will likely entail a good deal of research as well.  You might choose to sell goods or services in exchange for bitcoin.</p>

<p><strong>A more simple and practical way to earn small amounts of bitcoin is by completing tasks or surveys online.</strong>  There is an array of websites that offer this form of compensation - which will vary in terms of compensation level, accessibility, reputation, and reliability.  From my own experience, I can attest to the fact that www.BitcoinGet.com <em>did</em> pay out (which was my biggest concern).  Earning a full bitcoin through this medium alone would take quite a bit of time - but is the only option I can recommend that requires zero capital.</p>

<p>If you do want to earn some bitcoin in that manner, you should still create a bitcoin wallet and an address by following step 1 from @trollster</p>

<p>Welcome to Bitcoin!</p>
