As I've said before, Ancestry is the place to start just because there's so many records in one, searchable, place. You can join monthly and it's quite reasonable. As others have said, the trees are most often faulty and many are plain junk. However, I always look to see if there's ones that are well-documented as there are some good ones. Also, I look for trees managed by people who seem to have personal family materials (e.g. photos) not easily located. Those folks often had the classic grandma who had boxes of "stuff" and their trees can also be helpful. Usually, you'll not get responses to messages but serious researchers will answer so while my hit rate is low the answers I've received have been extremely helpful. In my personal research I made contact with several cousins who had and knew "stuff" and they, in turn, put me in contact with other cousins. In my cemetery research I've contacted family members of people whose stones I've repaired and got quite a bit of insider information (and even a few donations to our organization). So, Ancestry is worth it at the beginning; just be sure to check everything out.
I don't tend to use very many web sites but do use
newspapers.com, FindAGrave, and Fold3. All are owned by Ancestry and FindAGrave records are linked in Ancestry (but I've had better luck searching FindAGrave itself). For military records Fold3 can be helpful but often they only have the index cards. So, I pay the $ and order records - and definitely pension files - from the National Archives. The pension files are worth every penny as they contain lots of information you won't get from the service record; definitely don't pass on getting those if they exist. Once I know from things like census and marriage records where someone lived I usually target my
newspapers.com searches to those counties and time periods.
For things Civil War, this site is an excellent resource. The membership is populated by many who are experts in certain things and they tend to respond if asked. The dedicated sub-forums usually have the answers to common questions. This one is a good place to start for reference materials:
https://civilwartalk.com/threads/standard-civil-war-reference-works.84521/#post-647519
I do use carefully-constructed Google searches and those can lead you to lots of good materials, many digitized and downloadable. If I'm researching things or events, and not people, I use Google a lot. If it's Civil War related I always look in my references - I have about 90 books - and also peruse the bibliographies for the most likely useful source material. If there are books I determine which are the better ones and either buy a good used copy (one can often find such on Amazon) or get a copy through ILL. Again, I'll usually examine the bibliography to see if there's something I've missed. JSTOR is a good place for journal articles. You can see some for free but a paid membership is needed for most. However, If I find one I really need I can get a copy through ILL.
I also always look to see if there's an historical society or a genealogical society in the county where a research interest lived and, if so, contact them. Hit rates aren't high for those (usually staffed by older volunteers) but sometimes they can be gold mines. Libraries of record are also good places to look, especially if the research interest lived in one of the older eastern states like Virginia or South Carolina where there are very large libraries with all sorts of things not commonly available. All have search engines and almost always will make digital copies of records (for a fee). On occasion I've even hired a researcher to go to a library and search for specific documents. It's not cheap but only once did I feel I wasted my money.
I've used a lot of other sources, too, such as state archives and probate records and the Library of Congress (which has a lot of newspapers).
Research is usually a matter of starting with the low-hanging fruit and then following the bread crumb trails. It's a bit of an art, methinks, and one gets better at it with time. Also, one learns what types of things to look for in specific instances, where such things are most likely to be found, and who to contact with factual questions (hint: reference librarians are your friends).
Tenacity is a good trait to develop, too. If I'm working a project I tend to dedicate a certain number of hours each day and keep good records of what I've found, possibilities to look into, and missing pieces needed.
That's it off the top of my head. Hope that was maybe a little of what you wanted (I know you said you wanted on-line resources and a lot of what I noted aren't such but only a small bit of what's available is on-line).