こんにちわ!

はじめまして! わたしわ はマグダです
よろしくおねがいします

Here's (almost) exactly what you wrote, but with kanji in addition to hiragana.

こんにちは！

はじめまして！私はマグダです。

よろしくお願いします。

Something I noticed was that you used わ at the end of こんにちは. Generally using は instead is more correct, but I've seen わ used tons of times online by Japanese people on forums and whatnot, so I chose not to make a correction. In less casual settings, however, I recommend that you use は. Nice composition!