Hello,
Suggestion is to follow this Approach
Step1:
Pls playthrough and understand all the inbuild options available you can try in MOSS as a Power User or a Site Admin.
Step2:
Based on your Sharepoint Server Version
Install the related designer may be Sharepoint designer 2010(which support MOSS 2007 as well as 2010 Sites) or Sharepoint Designer 2013
for eg:you can get the designer 2010 for 32-bit from here
http://www.microsoft.com/en-in/download/details.aspx?id=16573[
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Step3:
Now you can start your customizations based on business demands:For free video tutorials you can use youtube ,embedded other useful links for reference
http://office.microsoft.com/en-in/sharepoint-designer-help/get-started-with-sharepoint-designer-2010-HA010370548.aspx[
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http://sharepoint-videos.com/sharepoint-designer-2010-fundamentals-training-course/[
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http://www.c-sharpcorner.com/UploadFile/40e97e/sharepoint-designer-20102/[
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All the best:)pls write back if you need support or stuckup in any place on how to use Sharepoint Designer.Happy Learning!