I recently learned according to the Torah ("Shema Isroel, Hashem is our God, Hashem is one"), that G-D is one and single. Please help me understand this unity.
The Shema conveys three fundamental Jewish ideas on the Nature of G-d.
First, that G-d is the only G-d. There are no other gods except Him. He alone created everything there is, and there was no other power or force present in the beginning. Only Him.
Second, that G-d is a unity. He is not split into parts, or personalities, or attributes. He is complete and indivisable. He is everywhere - everywhere. G-d is not limited by time or space.
Lastly, that to G-d alone should you offer your prayers. No other thing may you beseech or use as an intermediary between you and G-d.
This is unarguably the basis of all Jewish belief. To deny any of these three facts constitutes heresy.