The first day of Gettysburg was bloody. 15,000 dead soldiers were scattered over the battlefields and it wasn't even the full battle. The South had won at every point. They only had one disadvantage; they had pushed the Union to the high ground.
The day before, Union troops had sent up two corps to secure Gettysburg. The morning of July 1, A.P. Hill (a Confederate general) sent a corps to investigate who and where the Union troops were. Early that morning both sides had troops marching into Gettysburg from opposite sides of town. The firing started when the two sides met. Both sides had a steady follow of reinforcements during the day.