Blender how to reduce polygon for noob

Here is how you can reduce polygon on Blender

1. select your object
2. On the right hand side, find Object Modifier -> Add Modifier -> Decimate
3. Then just put 0.5 to reduce 50% of your polygon


If you got error "Non-manifold mesh as input"

Here is how you fix it
1. Select the model, press "TAB" to enter edit mode and choose the menu option Select -> Non-Manifold

2. On Object Modifier -> Add Modifier -> Add Edge Split Modifier, set the edge angle to 90 degree then press apply

3. You will have 10% chance to fix it this way

But 90% chance you won't fix it,

So what is the next step?

You need to get Meshlab on the link below
 http://meshlab.sourceforge.net/

It is a free open source software

STEP
1. Download Meshlab
2. Open Meshlab, import your .obj Mesh
3. Filters -> Remeshing, simplification and reconstruction -> Quadric Edge Collapse Decimation