Mike is quite right; the common practice is to take your (in this case, English) name and reproduce it using Japanese graphemes (kana). Many English phonemes are non-present in Japanese, and so the Japanese equivalents are used.
Technically, none of the syllables in your name are present in Japanese (assuming it's pronounced the way I say it; I'm from California). Because of this, the closest approximants to "Prescilla" would be what Hiroyuki Nagashima wrote: プリシーラ (purishiira). This word has no meaning in Japanese, it is simply the way a Japanese person would pronounce your English name.
Some people do take the meaning of their name and translate it into Kanji (which rarely results in a commonly used name, and very rarely if ever sounds like your real name), and some people even find kanji that corresponds with the Japanese transliterations for their name (such as 有道出人). These names technically have meanings, but are not "real Japanese names" either.
So I guess the question is, which method do you prefer?