What is the Amazon API Signiture

I am attempting to test the Amazon ItemtSearch API, yet it keeps sending back the error:

<ItemSearchErrorResponse xmlns="http://ift.tt/1ldHUXV">
<Error>
<Code>SignatureDoesNotMatch</Code>
<Message>
The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.
</Message>
</Error>
<RequestId>9a355851-98f3-42fc-8bc3-b795030b4510</RequestId>
</ItemSearchErrorResponse>

With the URL: http://ift.tt/1zb16xr(access key)&Operation=ItemSearch&Keywords=Rocket&SearchIndex=Toys&Timestamp=2015-04-28T23:20:38Z&Signature=[request signiture]

I have supplied the AWS Access Key as from the security credentials page, yet do not know where to find the proper Signiture parameter. I had used the secret access key but that did not work and simply threw the error as seen.

What signature are they referring to in order to get this request to work?