When you became a prime minister what can you do after?

It's unclear whether you mean after you have stopped being Prime Minister, or as soon as you become Prime Minister.

If you mean during the term of a Prime Ministership - there are a whole lot of things that you can't do, according to whatever national constitution you're talking about.

If you mean, after a Prime Minister finishes his or her term - there is very little restriction on what you can do, since you are simply a private citizen. Many countries, such as Australia, pay former Prime Ministers' travel and office allowances, so they can do things like compile memoirs and give speeches in various places.

Former Prime Ministers will sometimes make contributions to politics, but in general they are expected to stay of day-to-day political questions.

Many former Prime Ministers make careers in other areas such as business, for example.