What should I do in Tarawih prayer if I intend to pray the Shaf' (even-numbered raka'at) and the Imam is praying Witr (odd-numbered raka'at)? And should I add a raka'ah after the Imam says the taslim? And should I say "Ameen" to the Imam's supplication in that prayer?

It is prescribed for whoever joins the Imam during the Qunut of Witr to say "Ameen" to the Imam's supplication, and then to perform the Witr prayer after the Imam says the taslim (salaam); because he did not have the intention for Witr (when he joined), and Witr requires a specific intention. It is permissible to perform Witr with a single rak'ah after the Imam says the taslim, and what you did by praying Shafa' (an even number of rak'ahs) with the Imam is sufficient for you. It is preferable that before Witr there be two, four, six, eight, or ten rak'ahs.