Use the definite article, for example "the (Apple) i-Phone" when talking in general about the product. Use the indefinite article to refer to one individual product: "I have an i-Phone." After identifying it, we use the definite article or possessive: "My i-Phone is black." Note that if the item is countable, we cannot use the null-article (i.e., we must use an article or other determiner).

Both 'Apple Watch' and 'The Apple Watch' are possible. ie. You could both say, "I'd like to introduce Apple Watch" or "I'd like to introduce the Apple Watch". The first is a more general introduction, the 2nd by using the definite article 'the' is a more specific or focussed introduction. It's just a choice of nuance in this case.

@Gregory Thank you for the comment. That's what I always get when I ask questions about articles.

@Haley Thank you for the comment. I was wondering the company, Apple wrote "Apple Watch" or "the Apple Watch" in their official product descriptions.

After reading the comments below, I must add the following. Usage of articles in English is not always easy to define by rules. Much, if not most of the usage is simply idiomatic. The more you practice, especially reading current literature or news within the past 25 years, the more you will get a "feel" for when to use the article and when not. In other words, my advice is not to try too hard to memorize the rules, because the rules will drive you crazy. Just practice, practice, practice!