1) Just think about what type of character you want to introduce and think of a suitable way of introducing him, kinda like how in an action movie you always introduce your hero as a ''cool guy'' or if it's a romantic comedy or whatever you show your character doing wacky things because they're loveable and whatever.
2) it's perferable that all things should be part of the plot, or else you have filler, or disjointed storytelling, this does not apply to foreshadowing another possible story, however.
3) Building character is telling a story, and stories are usually told by characters actions and reactions, aka how a character behaves is extremely important to the story because what makes him/her happy/sad/angry is going to be what moves the plot along
4)Just have the ''Hero's Journey'' more or less in mind when writing, it's best to have each scene be as dense as you can make it
5) I'm indifferent to cursing, characters can be angry/happy/sad without resorting to cursing, it also depends on the age group your making this for, either way it's best to not do it too much.
6) Pretty much everything has to revolve around the main plot, a subplot is there to mainly develop a side-character or character behaviour who just couldn't go along with the main story traditionally, in anyway by the end of it it all must fall back on the main plot and influence the status quo like anything else that's happening
7) Subtext means quite a lot, body language, even in comic form can tell you all about a character, so do angles, scene blocking, character postures and placement all can pretty much carry a story on its own, but you have to gather yourself a lot of reference to understand how it works and how it should be done, I personally learned it from watching and studying tons of movies.