No guarantee for the correctness of the explanations given below as I am not a native speaker either but that's how I understand it:
- The first thing I will do is
......find a job for myself. = general statement, that's what you want to do
......to find a job for myself.= I don't think this is possible, I would never say that myself (no real explanation, more intuition)
......finding a job for myself= stress on the actual act of going out and looking for a job.
- My aim is
......find a job for myself. = I definitely don't think this is possible
......to find a job for myself. = I'd go with this one because you need either infinitive or gerund and I don't think this calls for a gerund --> change the sentence and the infinitive makes the most sense: "It is my aim to find a job."
......finding a job for myself. = I don't think this is possible, either.
- What I really want is find/to find/finding a job for myself.
--> same explanations as first example
- What we are doing now is
......help the poor. = that's what we are doing now after e.g. a long discussion about what we could do to make life better on earth.
......to help the poor. = to help the poor instead of the homeless.
......helping the poor. = stress on the actual act of currently helping the poor
- What we have done is help/to help/helping the poor.
--> same explanations as in the example above
- What we have done is helped the poor. = This is not a correct sentence. You have to change it into:
"What we have done is we helped the poor." --> When sb asks you about what you have done to again e.g. help improve the earth compared to others who have also tried to help in this matter, you tell them that you helped the poor (compared to others who helped the homeless).