When you water anything being grown in a container, you always want to water thoroughly until the water runs freely from any drain holes. Then don't water again until the plants need it. Can't tell you how often - it depends on the potting media and the size of the plant and the container. In my cool but dry summers, that's every 4-6 days with a 20G container. Just don't allow the potting soil to dry completely.

I only grow one tomato In a contaier in my kitchen garden. It's for snacking on. I use a 20 gallon party tub. How much I water depends on the weather. Sometimes it's more like a good soaking till the water runs out once or twice a week. But usually somewhere in the summer we get a hot dry streak for a few weeks and I might be watering every 2-3 days if the tomato is in full fruiting mode. I don't water quite so much till it runs out then- I'm just making sure I'm keeping the moisture amount even to help prevent skin issues like toughing or cracking skins. And since at that time I'm hitting up and checking the other containers there, I check the tomato to see if it could use a drink or top off.

I should note that I tend to use a mulch on top in my pot too. Usually a leaf mulch, though sometimes I've used pine straw. A bit of mulch helps a lot with keeping the pot even.

Since you're just starting you might want to invest in a simple moisture meter. I use the one pictured below, it is basic, about the cheapest out there, and they work. Lots of people sell this model with their names on it, rebranded, etc.,...it's a mass produced meter....eBay, Amazon, Walmart, corner hardware store maybe. It's not precise but easy to use and "close enough" to keep your plants growing nicely. Other than that, stick your finger down into the dirt a couple of inches...if you sense moisture with your finger, don't water. If it's dry at the 2" depth then water the plant. Personally, my fingertips are rather old and a little tough and can sense moisture/temperature as well as they once could...the moisture meter works better. And, with the meter, I can measure at all depths if I want to...at least to a depth equal to the probe length (roughly 12"). For outdoor plants in containers I do water until the water runs from the drains...but, these containers are in an area that it doesn't matter whether water spills on it or not. Best wishes.