Excel function: IF a cell contains certain text, then a specific formula executes

Help would be appreciated guys :)

I'm creating a ledger and I'm a little confused on the IF,THEN function. My dilemma goes as follows:

Function for Cell E1:
    If cell A1 contains the word "BUY", then execute the formula: "B1 * C1"
        OR
    If cell A1 contains the word "SELL", then execute the formula: "(100 - B1) * C1"

Note: Cell A1 will contain ONLY one of two words, "BUY" or "SELL"

Any help would be appreciated, this is like Greek to me :/