Hello Eiram and welcome, this is a good place to post your query.
It is not surprising that you find a words written in 1749 difficult to turn into sense, the use of words then so often differs from the modern meanings. Don't let it put you off Tom Jones, it is funny and cheeky. Do persevere with the language and enjoy the care with which it is written and its diversity.
I believe that opening sentence basically means " An author should not write in a specialised way, or as a charitable gift to only a few select people who can benefit and appreciate, but rather write for all to enjoy."
Seems to be excellent advice