Inside your shader, you have a texture sampling function (tex2d, iirc). It takes two parameters for a 2D texture, called "uv coordinates" which range from 0 to 1 and encompass the whole texture. So (0, 0) is the top left pixel of the texture, and (1, 1) is the bottom right pixel. These uv coordinates are defined for each of your geometry vertices, and interpolated over pixels that way. (you can also define what happens when you sample the texture outside the [0..1] range, for instance it can either tile, clamp, or mirror).

If you only want to sample, say, the top-left quarter of the texture, then you need your uv's to be between 0 and 0.5.

If your geometry was a simple rectangle, then you'd have those vertices with the indicated uv's:

The whole texture is displayed:
[source lang="java"]
(0, 0) ---------------- (1, 0)

| |

| |

(0, 1) ---------------- (1, 1)
[/source]

The top half of the texture is displayed:
[source lang="java"]
(0, 0) ---------------- (1, 0)
| |
| |
(0, 0.5) ---------------- (1, 0.5)
[/source]

The top left quarter of the texture is displayed:
[source lang="java"]
(0, 0) ---------------- (0.5, 0)
| |
| |
(0, 0.5) ---------------- (0.5, 0.5)
[/source]

These coordinates are often set inside your 3D modelling programs (you might have heard the term "uv-unwrapping" before, it is a method to "wrap" a model with correct uv coordinates so the texture doesn't get distorted), but you can also set them in hardware.

There is no easy way to just say "render the texture between pixels (20, 13) and (100, 49)" in a shader, because the shader doesn't care about what size your texture is.

k soo... I would have to create separate geometry for each part of the texture i want to use?

Yes, in theory, a 3D model is made of lots of little triangles, and each triangle has its very own "texture chunk" delimited by the triangle's vertices' uv coordinates. But this is usually automated through 3D modelling programs like 3ds-max, maya, etc... so you don't have to worry about it, you just read whatever uv coordinates were provided with your model and use that in your shader.

What are you trying to do exactly?

If you're doing animations, such as from a sprite sheet, you don't necessarily need to use a different piece of geometry for each sprite. You can modify the texture coordinates that are output from the vertex shader according to some shader constant (which would define the cel of animation you want).

Just add a constant value to each one.