a) The following listings are summaries only and it is imperative that you always check the rules and terms and conditions on the competition website to ensure both you and your story are eligible to enter. For example most competitions exclude previously published or awarded stories and some have regional or age limits.
b) Make sure you follow all the entry instructions including those for formatting.
c) Proofread and edit your story before entering it. Make sure you check spelling and grammar.
d) Reading your work aloud is a helpful editing tool. Perhaps even have a friend or family member read it through; a second pair of eyes often sees what the first misses.
e) Some competitions publish non-winning entries on their websites or in anthologies without remuneration. You may be quite happy to just have your story out there, there is nothing wrong with it, but be aware that story is then 'published' which precludes entry into most other competitions.
f) Read previous winning entries if they are available. They will give some insight into what is sought, although many competitions have different judges with each passing year and each will have their own likes and dislikes.
g) Prices are correct at the time of writing but may change throughout the year. Ensure you check the websites.
h) If you want to brush up on your writing skills you find there are a plethora of free courses on almost any writing topic. You can check these out in the article 73 Free Writing Courses You Can Do at Home.
All that remains to be said is have a go and good luck.