Welcome to the forums!
Everything already mentioned is a good idea, the only thing I'll add is that I usually catch a few Betas in the river next to the Oakenstead household in Willow Creek. Fishing without bait will help, as that fish is considered a low tier fish and so you'll actually have a better chance without it.
If you want to make sure you have a chance of catching it, look at the fish swimming around the fishing spot... if you can see a fish, you can catch it, so if you can see a Beta it's just a matter of time and luck.