"I want sushi."
ga marks the subject of the sentence. That makes it sound really weird when translated to English; a literal translation would be "sushi is desireable (for me)".
Japanese says some things very differently than English, that's just you need to get used to.
Because 欲しい「ほしい」in Japanese is an adjective (meaning desired, wanted). You only use を along with verbs. Basically it works like 好き X likes Y→(Xは) Y が好き(です/だ) X wants Y →(Xは) Y が欲しい(です)
Because "to want" is a static action, you don't do something phisically, for example "to like" is also a static action (sushi ga suki desu). if the word means, that you will do something phisically, then use "wo"
Honorific. You will the 'o' popping up in front of many things example おにぎり, おはな, etc. But beware of getting confused with words which already have an お in the beginning which may not be a honorific.
Why ga instead of ha? It's it because wsnting sushi is the topic not sushi itself?
ほしい is an adjective (sushi is desirable. It's a different grammar structure than in English) so sushi must be followed by が and not は. I hope this helped :)