How do i activate a Texture Pack?

SilverKytten explained about right!

Just Convert with ''mcpatcher'' and chose it

First enter on minecraft them go to options there is a option. It says : Texture packs

click on it then there you go

if you wanna know how to install a texture pack here are three steps:

Step 1:go to google and download your favorite texture pack.

Step 2: Go to start ===>run and type %appdata%. Find the .minecraft folder and go to texture packs folder.

Step 3: copy the winrar(the the texture pack) you got from step 1 in folder.

There you go.
Hope i helped

Once you have downloaded a texturepack from here, you can go onto miencrafct and go on the texturepacks section, once doing this you'll want to head to Open Texturepack folder, or soemthing like that, through that you find your downloads file and find the texturepack, then cut the texturepack into the original folder you started in - exit, then click it on Minecraft

Very simple - Open Minecraft, then go into "options"
There will be a button that says "Texturepacks," Open that
At the bottom will be a button that says "Open texturepacks folder," click that
Once that's open, drag and drop the ZIP file of the texturepack you've downloaded into it, and then close the page.
You'll see the texturepack on your list, now, all you have to do is click it and let it load, then click "done," and you'll have a textured Minecraft.

Do you not know how to get one or how to make it appear in-game?