What can I do if I was driving a straight lane when someone from the side came in font of me and I hit them?

Do about what? Get reimbursed for your property damages? You make a claim against the other driver with his insurance company and try to convince them you were going a reasonable speed, the other car had to yield to you as it was merging into your lane, by the time you could possibly be aware it would try to merge it was too late to avoid hitting the other car.