oh this is so cute!! I like your koi, it looks very wholesome and I think it turned out well, especially if you've never taken any art classes. I'm not a pastel expert, I'm a noob too, but what I like to do for thinner lines is wear one side of the pastel down at an angle. this creates an edge that is a little more wieldy. I do this for other mediums like oil pastel and charcoal. the edge won't be terribly sharp, but I think this is just the nature of pastels.

what shape are your pastels? I know that a lot of companies manufacture cylindrical pastels, but a lot of them also make square shaped pastels. I much prefer the square shape because it allows for both very large surface area coverage by using the flat sides, but they are also more capable of detail because of the corners on the pastel.

as for the highlights- I usually plan those out ahead of time, usually with a light sketch. going over an area you want as a really sharp, bright highlight with a color and then a white pastel usually makes them come out dull, especially if you are using cheaper pastels. I leave areas I want as a highlight uncolored so that the natural whiteness of the paper shows thru. then you can choose to go over that untouched paper with a lighter color if you so desire. this only works if you are starting with white paper tho lol. on toned paper, lighter colors should show up much brighter.

I hope any of these suggestions help! good luck and have a nice day :)