If you are having problem with a specific build, post a pic for help. Also highly recommend looking through the build of the month thread, which has lots of nice work in it. A lot depends on how high or how wide.
One thing to do is to build low on the ground first if want to work out the spans, less falling and easier to get to. Just raise it 1m off ground. Try burying the corewood a little into ground by hoeing up after placed the grid. So much is trial and error. Eventually your first build you are proud of will look like a dud.
For timber also remember you can clad stone for more height, double skin the stone to hide it.