I've been dabbling with bonsai since high school. Maybe 1970. I was entirely self taught, books mostly until Al Gore invented the internet. In 2003 I sat down and looked at my oldest tree, a pomegranate, that I had kept alive since maybe 1973, at the time 30 years old. It looked like shit. I decided that a 30 year old tree should look better than that, obviously I had missed a key point in my self taught education. So I joined a club that was active and had class, seminar and workshop opportunities. That has made a huge difference in my trees, some of my trees look like trees, rather than crap. There is a 3 dimensional element to bonsai that just does not translate to 2 dimensional media like books or internet.
Advice #1, join a group or get a teacher or both. I know $$ is an issue. If your local club has some senior members, you might be able to trade to get instruction from them in exchange for a little labor. Right now I would happily take on someone with a strong back if they would take payment in trees or pots. ( 63 with serious back trouble).
#2 Remember most books and videos are for trees in refinement. Very little, or relatively little is written about trees in early stages of development. Read the articles on Evergreen's website, they are all geared toward developing stock to become bonsai, this is a rare archive, most ''Bonsai articles'' are about near finished trees in refinement.
http://www.evergreengardenworks.com/articles.htm
#3. there are different stages to bonsai - seedling, young tree in development to become pre-bonsai. Pre-bonsai, Bonsai in that the tree has been styled at least once, Tree in Refinement - a tree that has been styled several times, and is nearly ready for exhibition.
Into this mix you also have collected material, which instead of being a seedling, it is a rough collected plant that needs to get established and growing before becoming pre-bonsai or bonsai. Collected material, is an inexpensive way to add older trunks to your collection.
Each stage has different techniques that are appropriate. Use refinement techniques on a seedling Japanese black pine and it will forever have a skinny trunk and look awkward. Or die. Seedling techniques on a refined tree can set the refined tree back a decade's worth of development.
So to learn the totality of bonsai it is good to have a dozen or more trees in the seedling phase and the pre-bonsai phase, at least one or two in the bonsai developed enough to be in cycles of being styled. And at least one tree that is in final refinement phase. Seedlings and pre-bonsai are cheap enough. The last two will take saving up for, or trading labor to an older bonsai artist in your area.
Key is a diverse development range in your collection, so that you can learn all aspects of bonsai.
#4. Trunks won't thicken much at all once you put them in a bonsai pot. My 30+ year old pomegranate was a tiny cutting in 1971, I grew it in a small bonsai pot for the entire 30 years. By 2003 it was 16 inches tall and only 1 inch in diameter. That is 30 years to go from 1/8th inch to one inch. If I had kept the tree in a 5 gallon nursery can for the first 5 or so years, the trunk easily could have been around 3 or 4 inches in diameter. So do not ''confine'' your trees to a bonsai pot until the trunk is the diameter you want it to be in the finished tree.
#5 Horticulture is a science, the science behind bonsai. Do not go to ''Gardening books'' for science, go to the references for the agriculture industry. Land Grant universities have ag extension services. They also do serious agriculture research. Read their references for silviculture, propagation info for potted plant trade, and such. Don't count on garden books to get it right. For example a 10-10-10 fertilizer is not a balanced fertilizer, 12-1-4 is balanced to what a tree actually uses. And 12-1-12 for a nitrate containing fertilizer is correct because the extra potassium is needed to make the nitrate available to the tree. If a book tells you to switch to a zero nitrogen fertilizer in autumn it is just a garden book and has no science in it.
#6. What I like about the free Ryan Neil videos is that he does use the current horticultural science in his discussions of horticulture. You won't get that from Nigel Saunders.
#7. DO start trees you can't get easily at nurseries from seed. Starting now, you will have nice trees to work with in 10 to 30 years. For example I have been starting batches of Jack pine and American persimmon from seed, unfortunately I am 63, and wish I had started these projects 30 years ago.
There is a lot more, but I've typed enough.