Well, technically speaking, his first "guest" appearance would have been the very first time he took advantage of using puppetry in high school as a method of getting into television when he made his tv debut on The Junior Morning Show in 1954. (Hey, he had to be a guest on other people's shows and get "discovered" before scoring a show of his own). As far as what his first guest appearance *after* he had his own show with Sam and Friends, it's a bit hard to say due to lack of video recording equiptment and records but he no doubt appeared a bit on shows local to Washington DC and then as far as broader exposure programs go, it's known that as early as 1956 he had appeared on Arthur Godfrey and His Friends, The Will Rogers, Jr. Show and The Steve Allen Show but due to lack of dates on a lot of these it's hard to narrow down what was first. (Steve Allen was November so more likely one of the other two).