i am an amateur when it comes to playing guitars as well and if u take my opinion out of all the songs suggestions, knockin' on heaven's doors is the one i found easier. also, when i started, i did Good Riddance by Green Day 
, which was the first one i ever played on my guitar. the chrods are easy to switch and the strumming is easy, the only part u'd have to concentrate initially would be the guitar picking 
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