I don’t know if there is a “best” essential oil for soaps- it’s what you like. Some people use fragrance oils specifically for soap so it’s skin-safe, but I do use essential oils for fragrance. With cold process soaping, much of the aroma of the essential oil is lost so you will usually get a more subtle fragrance.
As far as using the calculator, the best way to do it is step-by-step, filling in the boxes as you go. Your batch size, for example, is done by measuring your molds. If you’re following a recipe from someone else, you can type in the recipe for practice and see how it works. We don’t yet have a tutorial, but it works the same way as the other soap calculators in that you put your numbers in the boxes, and it calculates the amount of water, lye, oils, etc you need.
For example, I tend to do small-ish batches, and I measure my ingredients in ounces. So I set it building it by percentages, and ounces as my measurement. To begin, you can select the default settings for superfat, lye concentration, etc. So this is how the start of a recipe looks:
If you want to play with this, pick a recipe from the database and then enter those numbers yourself so you can see how it works for you. When I first started making soap, I chose recipes from a source or two that were trustworthy, and ran them through a soap calculator for practice as well as to ensure the recipe was not lye heavy.
It does have a bit of a learning curve, but once you do it once or twice, it becomes super easy.