It's great that you're riding your dad's bike. YouTube is your friend for bike maintenance and instructions. Maintenance you'll need to perform routinely: clean/lubricate the chain, check and inflate tires, fix flat innertubes, and adjust brakes and derailleurs. You should attempt to learn each of these in turn - as they'll be necessary in the order I've given them.
Get a bike stand or loop some ropes or chains from your garage rafters so that you can get the bike up off of the ground while you adjust it. Visit a friendly local bike store or cooperative and ask questions about basic maintenance. Maybe they'll have a class or guided sessions. They can recommend materials that you'll need (rags, cleaner, lubricant, tools). The rest is up to you.
If any of us were to write it down here, it would fill a book. Literally.
Good luck.