Many citation managers have extensive BibTeX support, and will do the majority of the heavy lifting for you in terms of generating the .bib file. Each citation manager has its own pros and cons, and you can see an extensive list of citation tools (both paid and free) and their features, including whether or not they support BibTeX, at Wikipedia.
On rare occasions, such as in the case of special characters, you may need to edit the file with a text editor, but most citations can be formatted automatically without requiring your intervention.
Overleaf is an online software that has numerous features, templates and other goodies such as command autocomplete and suggestions, that is available free from Purdue. A very useful page on the Purdue Website is found here with the sign-up link.
There are also numerous pages on how to learn LaTeX on this page which may be useful for the starting user. BibTex is automatically implemented into the Overleaf interface, but you will need to make a new .bib file and reference it as you would in a standard LaTeX/BibTeX setup.
More complex LaTeX/BibTeX guides for intermediate to advanced users.
BibTeX templates for common styles: