Learn Hardcore Russian: Вечером я собираюсь скачать приложение для учёта расходов. - In the evening I’m going to download an app for tracking expenses.

для учёта: учёт → учёта (genitive singular) = “for (the) tracking/accounting”
расходов: расходы → расходов (genitive plural) = “of expenses” So the whole phrase means something like for expense tracking / for keeping track of expenses .

Вечером is the instrumental singular form of вечер. Russian often uses the instrumental to express time when something happens: утром (in the morning), днём (during the day), вечером (in the evening), ночью (at night).

Does вечером mean in the evening in general, or tonight?

It can mean either, depending on context:

Вечером я… often means tonight / this evening if you’re talking about your plans for today.
It can also mean in the evenings (habitually), but then you’d usually add context like по вечерам (in the evenings, regularly).

What does я собираюсь literally mean, and why is it reflexive (-ся)?

Собираться means to get ready / to plan / to intend. In this meaning it’s used reflexively (собираться) and is followed by an infinitive:

Я собираюсь скачать… = I’m planning / I intend to download… The -ся is just part of the dictionary form for this meaning; it doesn’t translate as “myself” here.