Learn each kata in short sequences, once you know the sequences then put them together.
Try to visualise what you are doing and why, this might be explained by your Sensei or a senior student.
Key to understanding is knowing why or the effect of what you do.
Don't worry if other students seem to be ahead of you, karate is a journey not a race. Relax and enjoy your karate as everyone learns at a different pace.
As you gain experience you will be able to memorise katas and combinations quicker.