Late sixties, still at school, living on pocket money, I used to buy sampler LPs released by various labels cheaply. It was a good way to get immersed in music as a poor schoolboy! These probably constitute my first five though its a long time ago.
Picnic - A Breath of Fresh Air, Harvest label (Floyd,Deep Purple, Kevin Ayers, Edgar Broughton Band etc)
Fill Your Head with Rock, CBS (Santana, Chicago, Flock, Janis Joplin, Johnny Winter etc)
The World of John Mayall Decca sampler
Nice Enough to Eat, Island label (Free, Nick Drake, Mot the Hoople, Jethro Tull, King Crimson etc)
Gutbucket, Liberty label (Captain Beefheart, Groundhogs, Canned Heat etc)