When it comes to posting online DPI doesn't make a difference. 72 dpi will look just as good as 300 dpi on a screen. So it comes down to what you're working at, scaling it down later to post online, and what file type you save as.
To quote @EnzoCUEK on his site http://makewebcomics.com/article/web-vs-print-resolution/
"Technically speaking, however, DPI refers to the amount of ink (dots) your printer can fit onto an inch of paper."
(Visit the linked site for more info, there's a ton of great advice there)
Another thing to look at is what file type are you're saving as, jpg, png, gif, other? And what settings are you using with these file types. Oh look, Enzo has an article on this topic too!
Now, depending on what you're making you can just draw and create vector comics in Illustrator OR Manga Studio (I recommend Manga Studio). With vector images you can scale up or down, left or right, do anything to the image and still have crisp clear lines. I draw my comic Oops Comic Adventure (and everything I draw now) using Manga Studio's vector layers.
However, working larger in Photoshop and scaling down later has better results than trying to scale up. Raster images from Photoshop never scale up without losing a lot of quality.
You want to make sure, when you're ready to scale down to post online, that your canvas is at the most 940 px by 4000 px (can be less than that) with at least 72 dpi (you don't need more than that for posting on Tapastic).
In Photoshop when you want to save an image for web use, you can use the hot key ctrl+alt+shift+S or go to File-Save For Web, and a new window will pop open with a bunch of presets for saving online. I tend to go for JPEG High preset, however you can always experiment with the settings and see how it'll look in the preview window and how larger the file size will be. For Tapastic the file size must be less than 2mb.
