What is the conjunction in the sentence since i am so tall, i signed up for basketball.

The conjunction in this sentence is "Since". The conjunction is a part of speech used to join words, clauses or phrases. In this case, the subordinating conjunction "since" is linking two clauses together and it also functions as an introductory word of the dependent sentence.

Another characteristic of subordinating conjunctions is that after the clause, a comma should always be used as in this case there is a comma after word "tall" and before the word "I".