If we are sleeping alone at night and if we find our undergarment wet in the morning, may be because of shaytaani waswasa,
1) How do we perform Ghusl, what should be our niyat? Ghusl e Janabat or something else?

2) How do we make the clothes paak? Can we put them in the washing machine?

1- Semen is recognizable, as it is thick and sticky. Hence, if it is not like that, then probably it is not semen, so, you do not need to perform ghusl.
However, if you are in doubt, and you want to perform ghusl, then you need to perform ablution as well.
2- If the washing machine washes two times with pure water, with spinning in between, then it is OK.