Learn Turkish: Bunlar benim kişisel eşyalarım.

It takes the noun kişi (person) and adds the relational suffix -sel. This creates the adjective kişisel (personal), meaning something that relates to a person.

Why does kişi take -sel instead of -sal?

Like many derivation suffixes, this one follows standard 2-way vowel harmony. Since the final vowel in kişi is a front vowel (i), it takes the front-vowel version of the suffix, making it -sel.

The root word eşya is originally an Arabic plural. Why does it take the Turkish plural suffix -lar here?

Although eşya is the Arabic plural of şey (thing), Turkish treats it as a singular noun meaning item or belonging. To talk about multiple items, you must add the standard Turkish plural suffix -lar to make eşyalar.

Do I need to include benim to show possession in this sentence?

No, you can drop benim and simply say Bunlar kişisel eşyalarım. The possessive suffix -ım on eşyalarım already shows that the items belong to you. Using benim here just adds emphasis, as if saying These are MY personal items.