Geoff
To elaborate on the reply from Andres, transpose means change key and can apply to either
A) …changing the key of the music by re-writing it in the key you desire. So it’s written in say Eb, and you completely re-write it in the key you want to play in, maybe C. Some really experienced musicians can do this in real time in their head, without writing it down, but at your stage even attempting re-writing it would be a step too far.
Or
B) playing it in the written key while adjusting the instrument to sound in the desired key. This is what Andres referred to and is not what you need because you still end up playing in the difficult written key.
At your stage you probably should buy books that have a selection of songs in easy keys ie mainly C, F And G. These are available. If you are truly a beginner it is going to be a long time before you can comfortably play in a variety of difficult keys. This is true even on the arranger where you need to play LH chords to drive accompaniment. You would need to learn the chords in every key, quite daunting for a beginner, and realistically years - not weeks - of practice. In fact some “older “ beginners might never get there.
Your best bet for early success on an arranger is to learn the 3 or 4 main chords in the key of C, and learn how to read a simple melody line in key of C. This will get you started making music, and in itself be difficult enough. Then move on to similar in keys F and G. Where you go from there will depend on you, your natural ability, and the time you dedicate. But even a talented beginner will find that getting competent in just these three easy keys will take a while. There are no short cuts other than the one you have already taken, which is to buy an arranger. The arranger is way easier than a standard piano or organ, but still not instant.
Mike