Learn Croatian: Želim sendvič i sok. - I want a sandwich and a juice.

In Croatian, masculine nouns are split into two groups: living things (animate) and non-living things (inanimate). Both sendvič and sok are non-living (inanimate) masculine nouns. For these nouns, the accusative case (used for the direct object you want) is exactly the same as the nominative (dictionary) form.

Where is the word for "I" in this sentence?

You don't need it! The verb želim already means "I want". In Croatian, the ending of the verb tells you who is doing the action, so pronouns like ja (I) are usually dropped unless you really need to emphasize who is doing the action.

The English translation says "a sandwich and a juice". How do you say "a" in Croatian?

Croatian doesn't use words for "a", "an", or "the" (articles). The word sendvič can mean "a sandwich" or "the sandwich" depending on the context. You just say the noun on its own!

What is the dictionary form of the verb želim?

The dictionary form (infinitive) is željeti (to want). It is a very common verb, and its stem changes slightly when it becomes želim for "I want".