Like is one of the comparision indicators and is always a preposition. It should always be followed by a noun / pronoun / noun phrase and it compares with another noun.
Ex - Like LSAT, GMAT is a CAT exam. Here we are comparing two exams LSAT and GMAT which are two nouns.
In some occasions Like means "similar to"
Ex - Satvik runs like a chetha. Here noun - Satvik runs with a speed similar to chetha.
As is little more complicated since it has dual roles sometimes acts like a conjunction and some times prepostion.
When acting like a conjuction(remember, conjunction(or connecting word) should always join two or more cluases)
a) You should work hard in verbal AS every member of this forum do. (Two clauses are joined by AS - acting as a comparision)
When acting like a preposition, it should be followed by a noun. But the difference here unlike LIKE is that the noun reered is not similar but exact.
Ex - As a forum member, I give my inputs to this question.