To get free shipping to the US for most items, all you have to do is spend over $25. This does not include everything though, as some special items will incur a shipping cost. Musical instruments that are heavy, things like upright pianos or big PA speakers can sometimes incur an additional shipping fee.
If you're unsure whether your item qualifies your best bet is to contact Guitar Center customer service directly, and you can find more Guitar Center shipping information here.