Monologues for beginners are tricky because you want something that showcases emotion without demanding a Shakespearean level of technique. A lot of lists throw 'To be or not to be' at you right away, which is a trap; it's so famous any mistake feels huge. I'd steer clear of the classics at first and look at contemporary plays where the language is more natural.
Something from 'Rabbit Hole' by David Lindsay-Abaire comes to mind, maybe Becca's speeches about grief. The emotions are raw and immediate, but the words aren't overly poetic, so you can focus on being truthful instead of wrestling with iambic pentameter. Neil LaBute's early works, like scenes from 'The Shape of Things', also have these intense, conversational monologues that are great for learning how to build tension just with your voice and subtle shifts.
For a slightly different angle, monologues from screenplays adapted to stage can work too. The opening voiceover from 'American Beauty', if you cut the visuals, is a masterclass in cynical, detached delivery that still has to hold attention. The key is finding a piece where you connect to the character's frustration or desire, even if it's quiet, because that connection is what reads as 'acting' rather than just recitation.