Actually I am surprised because all the answers are wrong. "Alacaktım" literally means, I was going to buy. It is a combination of future tense and past tense, "Gelecek zamanın hikayesi" in Turkish. As you might already know, "acak,ecek" is future tense verb-makers, and "-tı" is past tense-maker, with a consonant assimilation (normally -dı), "al-acak-tı-m", last letter "m" represents the hidden subject "Ben" meaning "I". I hope it is clear now.