The term is acrostic - a composition usually in verse in which sets of letters (as the initial or final letters of the lines) taken in order form a word or phrase or a regular sequence of letters of the alphabet.
To create an acrostic, follow these five easy steps (courtesy of poetry4kids.com):
1. Decide what to write about.
2. Write your word down vertically.
3. Brainstorm words or phrases that describe your idea.
4. Place your brainstormed words or phrases on the lines that begin with the same letters.
5. Fill in the rest of the lines to create a poem.
Generate an acrostic poem for your name using these online acrostic poem generators: