Lots of great suggestions here. If I may offer a more generalized answer, I think the key, once you’ve cleared the first hurdle of learning the prayers and the sequence of mysteries, is to find a resource (usually a good booklet or app, or any number of saints’ writings about the various mysteries) that lets you learn *what *the mystery is that you are supposed to be meditating: What happened, what did it signify, what does it signify in the larger context of Christian life, and what might it signify to me, today, with regards to my specific prayer concerns?
Eventually, if you pray the rosary regularly, you’ll become acquainted with enough facets of each mystery to fuel your meditation without needing to look to any meditation aids, although from time to time you will still want to seek some out, in order to expose yourself to new facets of the mysteries.
Even with such knowledge, though, *focusing *can still be a challenge. Good techniques that I’ve found work well at that stage are gazing upon an image of Our Lord while praying, or mentally ‘re-enacting’ in your mind the mystery as you pray your decade.