Researching Thailand right now. No advance VISA required. But the hot dry season lasts until June. 34° average temperature. The rainy season starts in June, and temperatures average 28° during that season.
For residents of the United States, not too many. But a fair amount do. India did. I needed to apply for a VISA before I left, and there they are typically good for six months.
Fill out a form and send it to the consulate or embassy of that country usually. With India, I needed to send a couple of passport photos as well. It depends on the country.