How to write I boast about myself that is in anglo-saxon poetry form?

You need to have a greeting, who your parents are and where they are from, probably at least two accomplishments, and what you plan to do next (accomplishment). To get the kennings just think of the sound something makes or something like that, and alliterations shouldn't be a problem. If you're stumped, you can look at examples from Beowulf, lines 250-258, 407-426, 632-638, and 2510-2515 in the complete version.