There are few listed things are Najis which you can see below:
Ruling 80. There are ten things that are impure [intrinsically]:(1)

1. urine;
2. faeces;
3. semen;
4. corpse;
5. blood;
6. dog;
7. pig;
8. disbeliever (kāfir);
9. wine;
10. sweat of an excrement-eating animal.
(1) Each of these things is also known as an ‘intrinsic impurity’ (ʿayn al-najāsah).
Now if these things come in contact with a Pak thing they make those Pak things also Najis.
Now how make oneself Pak if he become Najis due to having contact with above mentioned things:
In case of few a person should have wajib Ghusl example if anyone sees Semen either after having sex with his wife or during sleep or due to any reason he has to do Ghusl e Janabat and if he touched a dead body after becoming cold then he has to do Ghusl e Mass e Maiyyet ( both Ghusls Janabat and Masse Maiyyet methods are )but if anyone touched urine or stool of Dogs (with wet hands) Kafir, blood and Alchohal then he has to wash only that portion which came in contact with these Najis things.
Now about Doubts or when doubts gets its worst position ( that we call Waswas) so make sure waswas is the act of Shaitan and if anyone listen and follow according to his waswas it means he is following Shaitan.
So please leave any type of doubt come in your mind and just follow the Shariat ( Islamic Laws) as our Imams taught us and now our Marja e Taqlid have written in their Islamic laws book.