Ambato is a city in Ecuador.
There are many petrol stations located along the Panamerica Hwy (numerically named E35) in Ambato
The E35/Panamerica highway itself gets a little windy and branches wildly when passing through town, so it's ideal to tell your lovely driver that you plan on heading further south. Riobamba is the next large town along the Panamerica and mentioning that as your next destination lets drivers know exactly where to drop you (if you plan on sticking to the Panamerica.)
The Panamerica highway also splits in Ambato, so it's possible to catch direct rides to Banos de Agua Santo. E30 is the road you'll want to take if trying to get to Banos. There are many drivers going along this road, tourists and locals alike, so it doesn't take long to catch a ride. I do recommend walking a little outside of the town center for this one, just to avoid local traffic. Due to the popularity of Banos, it's quite easy to catch a direct ride flying a "Banos" sign.
As mentioned before, if you plan on sticking to the Panamerica, make it clear to the driver that you're planning on heading in the Riobamba direction and need to stay on the Panamerica before they branch off, otherwise you'll have a bit of walking to do. There are many petrol stations along the highway, as well as large-company, cross-country bus routes where you can easily flag down a bus if you're feeling desperate/it's getting dark/etc. The heavy presence of "official transportation" makes this route a little more comforting to a beginning hitchhiker, as it's quite easy to opt-out and still get to where you're trying to go.
Other useful info
Food: Along the Panamerica, at the southern edge of town, there are many bus stops (identifiable by large patches of gravel, stopped buses and food vendor umbrellas) where buses stop/pick up passengers. There are plenty of food vendors at these places that sell cheap and delicious meals, as well as packable things such as peanuts and dried fruits. It's a good place to grab lunch and stock up on water/snacks
Bathrooms: In South America you typically pay a small fee to use bathrooms, but I've found at the petrol stations in Ambato, politely asking the station attendant if you can pay to use the bathroom, they usually let you use it for free. Not crazy important, but it's good to know.