So, the next step is to find a tree(s). For most designs, you need a straight, healthy, hardwood tree with a minimum of a 60" circumference (19" diameter) for sole support of a treehouse. You should keep it away from the road, water, power lines, neighbors' sightlines, rocks to fall on and build it no more 6-8' high for small kids (like my 7 and 5 year olds). I asked permission of my neighbor nearest the eventual location, and he said "Sure!". I called up the building department in my town and they had no problem.
When you add all the qualifications up that leaves me with two oak trees (with 72" and 53" circumferences) about 10' apart. See my picture!