Please take this question not as an expression of partisan blood-lust ("Lock him up!"), but as an exercise in conjunctive probabilities.

Donald J. Trump may or may not have committed crimes during his tenure as President, during his campaign, or previously in his life and business career.

For him to actually serve time as a result of being found guilty, several things would have to be true, with each one roughly dependent on the last.

1) He would have to have committed a crime (on the generous theory that he will not be jailed if this is not the case).

2) He would have to be eligible to be indicted. This means that either

a) he is no longer President, or

b) it is decided that a sitting President can be indicted (a matter of legal controversy at the moment)

3) A prosecutor or grand jury would have to decide that he should be indicted.

4) He would have to be found guilty.

5) A judge would have to decide on a sentence that included time in jail.

6) The sentence starts before a pardon occurs.

Number 5 in particular may be quite a high bar, especially given the potentially incendiary impact of jailing a President or ex-President who won election and enjoyed the continuing support of a large percentage of the American public. The American political/judicial system has shied away from any such outcome in previous scandals. Richard Nixon resigned to avoid impeachment and then was preemptively pardoned; Bill Clinton was impeached by the House without conviction by the Senate, and did not suffer conviction on criminal charges (though he was cited and fined for civil contempt of court, had his law license suspended for five years, and settled a civil lawsuit out of court).