• You could use a configurable, modern letter class like scrlttr2. An example can be found, e.g., here. If you need more help, we need more information. In this case you should also add a minimal but working example (MWE) of what you already have. – Schweinebacke May 18 '17 at 11:55
• There are many letter classes in CTAN and templates and more templates here and there. Take time to search, compare, and then decide if you need (1) just use some already done (2) modify that cute template im some way ... or (3) start from scratch with \documentclass{article} \begin{document} Dear Sir, Sincerely, \end{document}. If you get stuck with (2) or (3), ask about your specific problem but showing what are you trying (that is, a MWE). – Fran May 19 '17 at 7:54