Starting any woodworking project requires good wood. Knots, twists and other damage to timber will mess up your project before you even get started. Here's what to look for when buying individual pieces of wood.
Get picky when buying wood (no matter what the grade) and don't just settle for what's on top. Inspect boards looking from one end to the other and note any warping, twisting or splits. Pass on those that aren't straight. Then look for knots or other irregularities along the wood itself. Damage from rough handling can also render timber unusable.
When inspecting plywood, again check for warping or bowing due to uneven weight distribution. Bent wood won't saw properly, so avoid it.
A good rule of thumb is to not buy the last bit of wood from any pile.
If your local timber store doesn't have any reliable stock, move along and check another store until you find what you need.
How to Pick Materials for a Workbench [This Old House]