If all you've basically done is put ARF's together, don't bother trying to get into scratch building now. Scratch and plans building can be a much more expensive proposition than ARF's or kit building. You need more tools and time. Start building kits first to see the type and thickness of wood is used and for what purpose and for the size and type of plane it is used in. Try modifying (bashing) a kit plane. Buy some plans and study them and try you're hand at building from them. There's no replacement for building experience when starting to scratch build. Search the internet and books and read up all you can on model aircraft design, you will definitely need to do some homework here, i don't think there is really any short cut, otherwise scratch building will get very expensive when you see you're newly designed and built plane try and auger a fence post hole or the wings fold, etc. The reward for it all though is creating something new and seeing you're own creation (maybe the only one of it's kind on earth) fly, which to me, anyway, was worth the journey.
Here's a link to my current scartch build project.
http://www.rcuniverse.com/forum/m_4452331/tm.htm