There are two types of them:
direct ones: mi, ti, lo, la, ci, vi, li, le
indirect ones: mi, ti, gli, le, ci, vi, gli/loro
However with MI, TI, CI, VI, with the meaning ME, YOU, US, YOU, you can not be wrong, if you use them either for direct object or for indirect one. Examples:
He loves me-Mi ama.
He loves you-Ti ama.
He loves us-Ci ama
He loves you (two of you)-Vi ama.
Their place are always before the verb, except in the imperative form when the prounouns are attached to the imperative: Bring me there-Portami là. With the modals they can stand either before the modal verb ot attached to the infinitive: Ti posso aiutare/Posso aiutarti? Can I help you.
Direct forms: lo, la, li, le are used as a direct object.
Leggo un libro-Lo leggo. (because the book in italian has masculine gendre)
Leggo una lettera-La leggo (letter is feminine)
Leggo dei libri-Li leggo (plural masculine)
Leggo delle lettere - Le leggo (plural feminine)
Forms: gli, le and not so often loro are used as an indirect object introduced by the preposition A and usually corrisponds to the english preposition TO.
I give money TO him. Do i soldi a LUI with clictic form becomes Gli do i soldi.
I bring roses to HER. Porto le rose a LEI. Le porto re rose.
The form GLI/LORO is TO THEM:
To them it doesn't seem right- Non sembra giusto a LORO becomes Non gli sembra giusto.
Hope this helped!