You must dump the BIOS from your own PS2 console (legal requirement). Do not download BIOS files from random sites – they may be illegal or infected.
Download AetherSX2 (free, open-source, safe) from its official website or Google Play. Avoid "modded" emulators.
Use a PC DVD drive and software like Imgburn to create an ISO from your original PS2 disc. Then compress it to .CHD or .GZ to save space (no performance loss).