When using POST, I would recommend transporting the strings in JSON-format. JSON Allows you to serialize and deserialize objects, arrays and strings. Can be serialized and deserialized at both ends, so you can transmit data in both directions.
You need a language to receive the data from the mobile app. I’d recommend either the language ASP.NET or my personal favourite PHP. They are both decent web-programming languages.
In order to run these services you need a web-server. There are alot of web-servers out there. The most popular and easy to use is probably Apache / httpd. (Nginx can also be recommended – but is more difficult to work with).