How to Increase Your Score in BangDream?

First, the conclusion:

Factors determining game score: character overall ability, band bonus, attribute bonus, character skill.

Character overall ability consists of multiple aspects: star rating, level, side stories, skill, star training.

First is star rating. The difference in overall ability between star ratings is significant, and star rating also greatly affects skill effects. For example, with the same score-up skill, a 4-star card can increase by 100% for 5 seconds, while a 3-star card can only increase by 60% for 5 seconds.
Level goes without saying; just max it out.
The first side story of a character card can be read directly; it consumes very few materials while increasing overall ability. The second memory side story is not necessary for beginners; it is more cost-effective to save materials for upgrades like instruments mentioned later. Consider reading it only when you have surplus materials later.
Regarding skills, there are types such as heal, score-up, judgment improvement, and combinations. Skill upgrade materials are precious, so prioritize using them on 5-star score-up cards (if unavailable, 4-star is acceptable). If a card has a skill that increases score by 100% for 5 seconds, and max level increases it to 100% for 7 seconds, then five cards would give 100% increase for 35 seconds. This score boost is very important in matches. Many songs are considered efficient not only because of short duration but also because skill activations occur during dense note sections, making the score increase highly effective.
Star training materials are even more precious; it's best to use them on 4-star and 5-star high-score cards. Don't waste them on just any card (you'll regret it).

If you are a newcomer unfamiliar with the characters in BangDream, you can look at the icon in the upper left corner of the card (e.g., a star with "P" or a purple rose pattern). These indicate the band the character belongs to. You can check the relationship between characters and bands in the game via Band -> Character Encyclopedia.
Generally, by the late game, players usually have a favorite band or use the event band for the current activity. Newcomers can learn about BangDream-related projects (e.g., anime, 3D live performances, fan edits) to find a band they like, or simply follow their preferences, or decide based on the cards they initially pull.
Assuming you have found your desired band (mixing bands is not recommended), click Map in the upper left corner -> CiRCLE -> Shop to purchase the corresponding band's album and poster. Initial purchase gives a 6%+6% bonus. Each subsequent upgrade of the album and poster adds another 1%, up to a maximum of 12% + 12%.
Similarly, Map -> Edogawa Instrument Store -> Shop allows you to purchase 5 types of instruments for the corresponding band. Each instrument increases the entire team's stats, not individual stats. When fully upgraded, the bonus is 5.5% * 5.
Now you can understand why mixing bands is not recommended: these instruments and posters can only be set for one band at a time. In a mixed band, some members will not receive the nearly 50% score bonus.

Similar to band bonus, you can notice the icon in the upper right corner of the card. There are four types: Red (POWERFUL), Blue (COOL), Green (PURE), Orange (HAPPY). Think of it as: band is a broad classification of characters, while color attribute is a finer classification.
You can purchase food (same attribute +14%) and decorations (same attribute +16%) at Map -> Cafe -> Shop and Map -> Meteor Hall -> Shop respectively. Upgrading food consumes the same materials as instruments: gold and attribute fragments, while upgrading decorations consumes Michelle red bean buns (bear buns).
Under the condition of same band, if you cannot assemble a full team of the same attribute, you can substitute with 4/5-star cards of other attributes or 3-star cards of the same attribute. You can test the scores yourself; the difference is not huge. For cards that will be replaced eventually, it's acceptable to use them temporarily.

When you have assembled a team with same band, mostly same attribute, and have upgraded instruments, posters, albums, food, and decorations to at least half of their max level, you should be able to achieve at least SS on song clears, meaning you can get the 100 star gem reward. If you can assemble a full team of same band and same attribute, and max everything out, entering the 290k legendary room in co-op should be no problem.
Upgrade materials such as gold and attribute fragments can be obtained through co-op (co-op gives much more materials than solo) , or via Settings -> Exchange -> Michelle Sticker Exchange for exchange. At the Get Stickers section of the UI, you can exchange duplicate cards you've pulled for Michelle stickers.
Bear buns can only be obtained through certain missions, event rewards, or exchanging event materials in the exchange shop. They are usually the top priority to exchange for in each event.

Same band (Roselia) same attribute (Blue COOL) ↓↓↓↓↓