This is a massive topic, and I'm personally not well equipped to completely answer it, just approaching the novice → intermediate transition myself (nor would I attempt to -- volumes of books by hundreds of authors have been written about this over hundreds of years), but maybe I can at least point you to a few resources on here that might help you with your immediate problems.
First, before that, a lot can be learned by experimenting. Not on your actual piece of course; but just get some scrap wood and go to town. You don't want to know how enormous my pile of scrap finishing samples is.
Anyways, from one beginner to another:
So from what I understand, this is the process of finishing wood (excluding the sanding part). Stain, then apply finish.
- "Finish" is the entire collection of things you do to your raw wood to get its surface to its final form, which may include stain if you use it.
And about sanding:
- It's a hot topic. More experienced woodworkers understand that constantly sanding between every coat of whatever is a bit overkill, even if products advise it in their instructions. Over time, I have confirmed this for myself. Sanding as a first step, however, is of course pretty useful and typical.
And the finish that I can choose from are shellac, polyurethane, lacquer and varnish.
There are others (e.g. paint, or various oils, like linseed or teak, and you need not put a stain on first if it gets you the looks you're going for), and even within those there are subcategories, e.g. oil- or water-based polyurethane, polycrylic, spar urethane/varnish, oil- or water-based stains, etc.
Also be careful with the term "varnish". Colloquially it's meaning has expanded and personally I find it to be confusing in conversation unless I make sure everybody is on the same page first. This page sums it up:
Varnish is an older type of finish made from resins, oils, and solvents, but very often, the term "varnish" is misused as a generic name for all types of wood finishing.
As for the ones you identify, the best I can do is point you to this nice little write-up. From my own limited experience, maybe this will help:
Shellac (dewaxed): A nightmare, never useful to me, not for the faint of heart. The only way I've ever figured out how to apply it smoothly is wipe-on, heavily diluted, about a dozen coats. It dries extremely fast, and doesn't really offer much protection against anything, although it does look nice. I went through a phase where for some reason I tried to use shellac for everything, mostly I was lured in by its fast dry times -- looking back, my success rate was 0%. The only thing I actually use it for now is coating the work surface of random MDF jigs to keep it from getting torn up; it's a pretty good "sealer" for MDF (e.g. my router table has an MDF surface, finished with shellac and a well-maintained coat of paste wax).
Oil-based Polyurethane: My favorite general category. I find it easy to work with and strong, takes a long time to dry and requires patience, but pretty versatile. I've learned a lot about working with it.
Polycrylic: This is easy to apply, not as good scratch resistance as a more "hard core" polyurethane but still great especially for shelves and stuff, and dries very quickly. Problem is it doesn't stick to oil-based stain well (I found this out the dumb, hard way). You could lay a coat of shellac down between the stain and the polycrylic, although that's a pain. Instead you could go for a water-based stain. A water-based stain plus a polycrylic finish is a great balance of protection + dry times and cleanup ease, IMO.
Lacquer / varnish: I've never really used lacquer. Varnish is still a pretty generic term, so if you count e.g. Minwax Helmsman as "spar varnish", that's about my experience there (incidentally, I'm a huge fan of that stuff on top of stain for indoor and outdoor projects alike).
Boiled linseed oil, Teak oil, "danish" oil, etc: I love the look of BLO in particular; it doesn't offer much protection but it's really easy to apply (and to repair) and can really give some beautiful results, especially without stain.
There's other random fun things you can do, too. For example, for a while I was experimenting with bleach or oxalic acid on poplar to remove the green tint, with various oils on top of it, with great results.
As for application methods you just kind of have to experiment. For stain, oils, and shellac, I like wiping it on with a rag. For polyurethane and spar urethane I'm a big fan of foam brushes.
Preventing brush strokes depends on what you're talking about. For oil-based stain I always apply it like so:
- Wipe on thick with a rag. Don't really care about evenness so much as long as you don't leave big drips. Primary purpose is to just get it on the wood.
- After 5-10 minutes wipe / gently buff out the excess.
- Let it sit for 15 minutes or so then repeat from step 1. Also shake / stir the stain periodically, sometimes it can separate and settle quickly then it's very light when you apply it.
For thicker stuff like poly I usually do some variant of the following (how many coats, how you sand, etc. really depends on the look you are going for and how much time you want to spend):
- Slop it on quickly with a foam brush, just to get coverage.
- Working fast, smooth out any drips on other faces (besides the one you're working on) as you go.
- After slopping it all onto a section (do this within a minute or two tops) run the brush in single, continuous, even strokes along the grain to smooth it all out.
- Repeat as necessary.
- Sand lightly at the end if you want. For glossy finishes I like to wet sand once then hit it up with ultra-cut car polish and a buffer (Ryobi makes a cheap but effective buffer that they sell at Home Depot). For non-glossy I've found that, after it's fully dry a quick single pass with 400+ followed by a quick wipe with a rag + water / mineral spirits to get rid of the dust smooths it out nicely.
For shellac; books have been written about application techniques. It carries a lot of traumatic memories for me so I won't get into it, although if you're feeling masochistic check out French polishing (there's way easier ways to get a nice shiny finish in modern times, though).
For paint I just cut in the corners and stuff with a brush then use a roller.
Anyways I've already typed more than I wanted to and I'm sure others here can answer this much more effectively than I can. If there are any take-home points from all of this I would say:
- Experiment on scraps.
- Decide on a look you want then try to figure out how to achieve it.
- Feel free to ask more specific questions here about any of this as you experiment.
- There are definitely multiple bad and multiple good ways to do things, but generally there isn't always a single Right Way™ for everything. There's a lot of cats and a lot of ways to skin them so you just kind of have to settle on whatever you like to work with, in whatever way works for you, that also gets you an end result that you are satisfied with.