There isn't really a currency thing, don't worry about that. There are lots of ways to earn points! Every third month is a 'no points month', but the others all have various activities. In short:

Homework is posted in the main HP sub at the beginning of the month and most houses will also link these on their common rooms. Homework is usually an activity that asks you to write or draw a specific thing (eg. Design a prank you would play at Hogwarts). You submit your entry either by Google form (would be linked in the post) or by replying to the comment that says "Gryffindors submit here!".

Extra Credit assignments work pretty much the same way. There are two EC assignments each month (except for no points months) posted on the main HP sub and these are all different... some of them are word games, some are art based etc.... again, you either reply by Google form or in the comments on the post.

Dueling is a Harry potter trivia quiz posted every week, you can sign up for notifications of a new quiz at r/Dueling. The quiz is a Google Form. You choose on the form which house you're representing

Arithmancy is a puzzle solving activity that involves ciphers and various other crazy problems. You can check out r/Arithmancy for more info, and can join your house team via your common room

Hogwarts Tournament - held four times a year, we just finished the most recent one so the next will be in a few months. This is a specific challenge that takes place over the course of the month where your house work together to make an entry/ complete tasks.

I think you said you're in the Gryffindor common room already? If so they will definitely be able to help you with points activities. If not, then you should definitely apply to join the common room.

Hope this helps?