A 'Charm' is typically an item that was worn by all manner of people for various reasons. The are charms for protection, love, money, vanity, even fertility. Often these items would be jewelry - as they were small and easy to wear often (they were very pretty). So to start, I would find an item you are willing to wear often, is comfortable to wear, and not very large. You may prefer a small bracelet opposed to a favorite shirt of yours. Rings work great too.
Second, you are going to want to figure out what you want the charm to be for. As with any spell, you need to realize your intent before beginning any sort of preparations. Directionless causes don't usually last.
Third, figure out your method. Whether it be personal, or one you are borrowing, prepare for the proper spell casting and/or ritual to create your charm. Follow all instructions down to the last detail. Tweak it if you must (trust your gut). Make sure to gather any herbs, candles, oils, incenses and other various tools/supplies you will need for the ritual/spell.
And after you have completed the ritual, you can now start wearing your charm (Or keep it on your person).
Before you ask, I am not going to give you a spell, I know you are perfectly creative enough to make one of your own, and I trust that by asking about this you know enough about your craft and by past experience you will know what works for you. If not, then, well, you will.
Worst thing that could happen is it doesn't work.