はじめまして、わたしはキャサリンです。よろしくおねがいします。
First of all, there is no particle "わ” in Japanese.
I know you made a mistake, that's because you actually know the rule that when you use は as a particle, you pronounce like わ.
Also about "ぼく", usually only boys (especially little kids. Most of guys use おれ) use it.
Besides you wrote in formal way which includes です、ます, so you should use わたし.
If you wanna write in informal way, you could use ”わたし、あたし、うち" since you are a girl.
Good luck! : )