For part one, if you browse on mobile, then you want Pleco. Far and away the best app for Mandarin learners. If it has to be on a PC/Mac, then Wenlin is a clunkier alternative.
For part two, if you use Pleco you can download frequency-ordered flashcards for it like these. Personally I’m not sure learning the 3,000 most common characters is particularly useful – you can learn 東 “east” and 西 “west” but when you see 東西 in running text you might just think it means “east, west” instead of it being a single word meaning “thing”. Instead you could download vocabulary files for the HSK or TOCFL tests by grade – they are very roughly clumped by word frequency, which will be a lot more useful than character frequency.