The most obvious, and second most common, spelling of [sh] is <sh>, as in ship, bushel, push. In a few words, mainly from French, [sh] is spelled <ch>, as in Chicago, brochure, cache. The rather rare <sch> spelling of [sh] occurs in Yiddish words like schmuck and schlemiel, and in a few others: schottische, seneschal, and schwa, for instance. In the mercifully rare botantical name eschscholtzia [sh] is spelled <schsch>!
Perhaps surprisingly, the most common spelling of [sh] is <t>, as nation, initial, and ratio. It is one of a group of palatalized spellings, in which a letter that normally spells a sound pronounced towards the front of the mouth is used to spell a sound pronounced further back in the mouth, against the hard palate. For instance, the <t> normally spells the front sound [t] as in native, but in nation it spells the back palatal sound [sh]. Other palatalized spellings of [sh] are <c> as in ancient, ocean and glacier; <s> as in dimension, sensual, sugar, and insure; <sc> as in conscience, luscious, and fascist; and <ss> as in mission, assurance, and pressure.