Hello, I am new to Bonsai trees and have always been fascinated by them. My goal is to make a koi fish pond and in the center have a beautiful Bonsai tree. It will be outdoors. I'm not sure what will be the best in my area. Please help.
This is a pretty tall order, involving two pretty complex things. It can take some time, depending on how you want to move ahead.
Koi are a story all their own. An outdoor koi pond can be quite an investment in time and money. There are some "koi folk" here on b'nut, but it's really a topic all its own. FWIW, if you want koi and not goldfish) be prepared for substantial ponds/filters/upkeep. Koi are jumpers and can and will jump out of small ponds. They're also on the menu for heron, raccoon, foxes and coyotes.
Bonsai is also a commitment of mostly the same in time and money. They require daily care and maintenance (not to mention time to develop). A choice for a pond might be a bald cypress--which can be grown directly in the water--but whose growth habits aren't really in line with the traditional image of bonsai.
It's best you narrow down what you're after--deciduous or conifer tree for instance? What ideal image are you after with the tree--there are many types of styles for bonsai, ranging from rugged tree shaped by mountains (conifers) to lowland trees (like maples) that present a more prosperous image. Both conifers and temperate zone deciduous trees will be hardy in your area--please add your location into your avatar. It can save us time in providing advice.
Since you're starting out, you will need to learn how to care for your tree consistently and effectively to shape it into a bonsai.