What happens if you have yelled at your dog; what should you do?

Give him some love and don’t yell again. Get yourself a book on positive dog training.

What if a dog was abused in its past, so it doesn’t trust people? How could I gain its trust and let it know I love it?

Patience, lots of love, and letting your dog come to you rather than you approaching it will go a long way. If your dog was abused, it was probably around a lot of loud, angry noises and sudden movement, so be as calm as you can around it and speak in soft, soothing tones. There are some good tips in Build Trust with an Abused Dog that can help.

What should I do when my dog is sick?

Take your dog to the vet. If the vet is closed and the situation seems serious, find your nearest emergency animal clinic.

How to take care of a pet without going to a vet?

That doesn’t really work. You need to go to the vet or just don’t get a pet. You could get fish or something that doesn’t really need a vet, but your animal’s health is important.