Begin with showing. That should be your default mode. It is better to write a "show" and delete it to replace it with a "tell", than vice versa. Telling is a kind of shorthand, a fact that has to be memorized, while a scene (though it may be a hundred times longer) is memorable, it is imagined.
"Joe is a violent man," is telling. A scene in which Joe is innocently bumped by a girl not watching where she is going, and Joe shoves her hard so she falls to the ground and tells her "Watch where you're going, bitch," and then walks away, that shows us Joe is a violent man.
You get into your character's head by giving her an event or scene she needs to react to. This does not have to be central to the plot, but preferably not a throwaway scene, maybe an incident you can mention again later, or other characters may refer to.
People are defined not only by their thoughts, but more importantly, their actions. She needs something that causes her to take action, or that causes her to speak: As a question, as a statement, as a warning.
Your character must be motivated by something, whenever she acts, whenever she speaks. This is not just something BIG she wants (she does need that), but this is the moment by moment motivation of what she is doing, to survive, to make progress, to learn something she needs to know, to take action, to get something from somebody (in person or conversation).
You get to know her by the decisions she makes to advance herself to a future that she wants, and/or escape a prison (metaphorical or real) she is stuck in.
Skip writing about the three days she did nothing. Skip writing about the airplane flight from Seattle to Miami. If nothing happened, we don't care. If it was a welcome rest, that's all you had to write.
Likewise, you as the author need a motivation for whatever narrative you write. You are telling it to convey something to the reader, but at least try to make sure that isn't PLOT information. This depends on style, but in general what you describe should be a setting, OR "in the moment" stuff like how something made her feel, or confused her. Give as little backstory in your narrative as you can get away with, as little "world building" as you can get away with.
If your sentences are "bland", it is (likely) because they don't matter to the reader. You have given them no reason to care. There is not enough conflict going on, and you need to open on a scene, with no backstory at all, with your character in it, doing something. It can be scene alone, or a scene where she is arguing with a clerk, or killing somebody, or trying to pick out a new refrigerator that she can't really afford.
You don't use this scene to reveal her whole character and backstory. You use it to reveal ONE THING about her personality (generally not appearance), and makes her a person. Preferably something not dead average, but in any case make it clear this specific trait is hers.
So brainstorm. What is the most unusual thing about her personality? What are the consequences of that trait, how would it manifest itself in some situation?
So she is an assassin. We may not want to open with a scene where she is killing somebody; that takes more setup than we can afford. But, maybe she is good at infiltrations, and disguises, and concealment. Perhaps she is at a shop where they sell stage makeup, and she is buying contact lenses that make her brown eyes blue, and on an impulse picks up a new product the clerk shows her to make realistic looking scars. The reader doesn't have to know she is an assassin, the clerk may ask her where she is acting and she tells him a lie, but she is not typical, a small percentage of girls would be delighted to buy some new scar makeup on a whim (outside of Halloween).
This gives us something to hang on, you've shown us an individual, this is a friendly but lying girl into some weird appearance deceptions, and we are curious about her motivations.
Now your MC is yours, I am just showing an example.
When to tell: If the showing of a scene is boring. It tends to be boring if there is a lack of conflict, if they do not matter to the main character. You can either delete them, or replace them with a telling. The best things to tell are things the reader will not have to memorize, do not expect them to remember ANY fact you give them, especially a fact about the trait of a character. But a "fact" they can forget, that's fine. What was for dinner.
"She took a taxi to the airport." This gets her from her apartment into the airport, it may be important to the reader, but unless the character is reacting to the difficulty of hailing a cab, or the driver, or traffic or something on the ride in some way that reveals a new aspect of her character (she's a bully, or she knows the driver is taking the long route and says nothing), it is a waste of the reader's time, and boring. A generic driver and generic taxi ride is fine, and we cover that transition in a sentence.
The reader expects if you are describing something in detail it is going to lead to conflict (and or a new (to the reader) experience in your character's life), and every time it does not, they do not feel "immersed" they feel cheated. If the author does not have a motive for including a scene (it must show something new about the character in some way) then the scene should be replaced by telling.