There isn't really a verb form. You say things like 'I am the goalkeeper/goalie for X...' or 'I play in goal for X.'
'To keep goal' is another phrase that is sometimes used. I keep goal for... But that sounds kind of formal, more like the kind of thing that would be used in writing than in speech. 'Guard' sounds extremely strange to me, I guess Americans use it because of parallels to sports like basketball and (American) football where that is the verb of choice. In a UK context that verb would not be used.