Eventually you will likely want to add some curves to your borders, but since you have such a big project staring you in the face, a thought might be to just plant what you currently have as your back border (tall perennials), bumping out where you might want trees and shrubs (which will want more space to accommodate their eventual mature size), then gradually increase the width of your beds each season, along with additional plants, so you don't get totally overwhelmed. In the meanwhile, a simple cut-in edge works well (just dig a spade straight down along your edge, mounding the soil into your bed so there is kind of a bit of a gap to discourage your grass from growing into the garden beds). Over the course of 2-3 growing seasons, you would have more experience, perhaps a better idea of what you want/like, and experiment with different layouts.
Do think about your hardscaping (as others have noted). Patio, firepit, benches or other sitting areas, fountains or other water features, garbage cans, etc. I'm a visual person, and it works for me to physically place things where you might like a finished product - set up some lawn chairs around an overturned bucket to visualize a firepit area, walk to and from the house to see how handy (or not) it might be, where will you keep firewood, etc. Set out your trash and recycle and see how hidden (or not) it might be to where you will spend your time.
Have fun with it and don't stress. Even a professionally landscaped yard takes 3 years to really come into its own. There is truth to the adage: plants sleep the first year, creep the second, and leap the third. Patience is a virtue.
