You have to buy the game to play it, thats the way games work, and no one would just hand out login details as they paid for the game, so please don't ask.
Speak to a parent and see if they will buy it for you, or get a paper round or something to save up for it. Its not that expensive compared to a most other games.
To create a username, register a Mojang account at account.mojang.com. After you are logged in, you can purchase a Minecraft account. You will be sent an email with a redemption link, which you can click to complete this process. If you do not click the redemption link, a green box will appear on your Mojang profile, prompting you to choose a new username, or import a previously-registered one.