Are you able to block with a creature that has summoning sickness? My friend always has at least 1 Moaning Wall in his hand and plays it each chance he gets, even when I attack and it has summoning sickness he tells me that creatures with summoning sickness can still block creatures.
Yes, you can block with a creature affected by summoning sickness.
This is the Comprehensive Rule about "summoning sickness"; I emphasized the relevant parts in your case:
302.6. A creature’s activated ability with the tap symbol or the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since his or her most recent turn began. A creature can’t attack unless it has been under its controller’s control continuously since his or her most recent turn began. This rule is informally called the “summoning sickness” rule.
As you can see, at no point the rule states that you cannot block with a creature affected by "summoning sickness".