Be careful before chanching your minecraft texture! Open the txt Pack first and inside is normally a txt file wich tells you how to install it. Before you do this, save your minecraft.jar and your world save to an extern folder. In cause of a crash you can relod it from there....

Now if you go into minecraft and click on "Textures and Mods" or what ever it is. The button that says "Open Texture Pack File" I think thats what it says anyway. Just put the Texture file in there.

but if you don't want to do this then put minecraft into the texture pack folder and just launch minecraft

if ur on a mac, then you go to FINDER, LIBRARY, APPLICATION SUPPORT, MINECRAFT, TEXTURE PACKS, and place them in there as a .zip file

if your on a windows os then
start -> run -> type %appdata% -> .minecraft -> texturepacks
from there place your texture pack(s) into there as a .zip file