The answer to your question would be that the transitional word that would complete best the sentence is the following one: However. Therefore, the sentence would be as follows "I went to the mall after school; however, I did not have much money to spend".
"However" is the best transitional word to complete this sentece because of its meaning and punctuation. You should use a semicolon before "however" when it introduces a complete sentence. Then, after such word you should use a comma. As you can see, "however" is followed by a complete sentence (I did not have much money to spend). As regards the meaning, the sentence states that the speaker went to the mall but he/she didn't have money to spend. The transitional word chosen is used when you say something that contrasts with a previous statement.
