Keep your text clean, with a minimum use of html code - I suggest only italics and bold - and then, only if you really must. Think mainstream publishing - how often do you see italics and bold? Not often.
I write straight into a .rtf file and, where I (rarely) use italics I use standard html e.g.: <i>text</i>, remember two returns at the end of each paragraph. Avoid "wall of text" - many people read on small devices, give them plenty of space around the text. After a while, you'll find yourself keeping your para lengths roughly consistent - for me it's a simple, visual thing, roughly what you see in this reply is my default para length.
When I upload an rtf file it doesn't give me a preview, so I've learned to scrub my raw text using every trick I've learned. Run spell check, run "'find text' reviews on all of your bad habits, proof read, edit.
Avoiding the desire to submit immediately. Many writers learn a set of self-edit techniques, but even the best of us never get 100% perfect copy. Something will always get through - but for everyone's sake, get rid of the stupid mistakes, be at least competent with your grammar, especially dialogue punctuation. Do NOT put any web-links in your text.
There is only one site editor, Laurel. She vetts every story, newbies can expect their early stories to take longer to check, because she will read more closely to make sure content complies with the basic rules (no sexual description for any character < 18; no rape unless the victim ultimately enjoys; no bestiality etc...). Newbie stories typically go up in 4-6 days, 2-3 days once you're established - if you are entering a competition it's usually quicker (comp entries get priority).
If a story get rejected, the site uses a set of standard reasons, usually couched as questions. Review your text, remove or correct any problem content, and resubmit. If you need to clarify anything, use the "Notes for Editor" box.
Good luck - and drop by here if you've got any more questions. Someone will help you.