if you are just writing prose and not doing anything fancy, you should be fine. Both the dreamwidth coders and the archive of our own coders do a good job making sure those two sites are accessible to screen readers.
Using semantic markup (strong and em instead of B and I) is a good technique, but all modern screenreaders actually do fine with B and I.
Things which are unlikely to apply with fan fiction in either of those two contexts, but maybe well:
As a general rule, use CSS to do fancy layout instead of tables. Use HTML headers instead of bolded text to separate sections. Screen readers will read all the text on the page in the order it appears in the HTML source (technically in the DOM, but for the layperson the best way to imagine this is to know that screen readers will read all the text in the order it appears in the HTML source). If you use CSS to move it around on the page in interesting ways, the screen reader will be ignoring the CSS.
Don't convey important information in color or punctuation. (Many screen reader users have their configurations set to ignore punctuation or silly HTML elements such as ♥ ("hearts").) If you use images, make sure that you have meaningful alternative text for each image. What would somebody want to know about this image if they couldn't see it? Don't use the TITLE attribute to convey information that is not conveyed somewhere else as well; this will be invisible to most screenreader and keyboard-only users.
If you do anything fancy with JavaScript, make sure you have talked to somebody about how to make sure it is accessible to both keyboard-only and screenreader users.
If you are on Windows, you can install NVDA, an open source screen reader, to get some idea from a beginners point of view what the process is like.
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Using semantic markup (strong and em instead of B and I) is a good technique, but all modern screenreaders actually do fine with B and I.
Things which are unlikely to apply with fan fiction in either of those two contexts, but maybe well:
As a general rule, use CSS to do fancy layout instead of tables. Use HTML headers instead of bolded text to separate sections. Screen readers will read all the text on the page in the order it appears in the HTML source (technically in the DOM, but for the layperson the best way to imagine this is to know that screen readers will read all the text in the order it appears in the HTML source). If you use CSS to move it around on the page in interesting ways, the screen reader will be ignoring the CSS.
Don't convey important information in color or punctuation. (Many screen reader users have their configurations set to ignore punctuation or silly HTML elements such as ♥ ("hearts").) If you use images, make sure that you have meaningful alternative text for each image. What would somebody want to know about this image if they couldn't see it? Don't use the TITLE attribute to convey information that is not conveyed somewhere else as well; this will be invisible to most screenreader and keyboard-only users.
If you do anything fancy with JavaScript, make sure you have talked to somebody about how to make sure it is accessible to both keyboard-only and screenreader users.
If you are on Windows, you can install NVDA, an open source screen reader, to get some idea from a beginners point of view what the process is like.