I'm really confused on kanji. Since the characters are from Chinese, are you technically learning Chinese as well? How many characters are there to remember, and what is a Japanese foreign word?
> Since the characters are from Chinese, are you technically learning Chinese as well?
Would you say that since English is written with Latin script, when you study English, you are studying Latin, too? In this case the answer is yes. Studying kanji will help you in case you decide to study Chinese later, but they are two completely different languages.
> How many characters are there to remember,
it depends on the level you want to reach. Take a look at https://en.wikipedia.org/wiki/Kanji_kentei to get an idea about how many kanjis a native Japanese learns at any age. If you are serious with your studies you should aim at least at learning the 2,136 kanjis native speakers learn in school https://en.wikipedia.org/wiki/J%C5%8Dy%C5%8D_kanji
So should I start the Chinese course to learn kanji? Just the start of it so I could learn the kanji characters and there sounds
Hi! No, Chinese and Japanese characters have completely different readings. The characters themselves are not the same, since China has had them reformed, and Japan mostly hasn't. Japan has also invented some unique ones. If you want to learn Japanese, use only Japanese-specific resources. Mixing the two languages is highly not recommended for people who do not come from the kanji areal. Good luck!
If you are interested in learning more kanji, I recommend one of the many kanji-focused resources that are available. Japanese kanji and Chinese characters are related, but not the same. Use resources that are specific to kanji.
Unlike hiragana and katakana, there are many, many kanji characters to learn, so you can expect that it will take you much longer to master them all. However, if you are interested in reading or writing Japanese someday, understanding kanji is ESSENTIAL. Many textbooks provide Japanese words in hiragana/katakana or romaji, but native Japanese sources will use kanji in almost every sentence and most words. It is best to get started early, studying kanji while you build up your knowledge of Japanese grammar and vocabulary.
WaniKani is a good place to start - easy to use and with built-in repetition, it builds up your kanji knowledge from the simplest forms to the most complex.
Kanshudo is another site that I have found helpful.
You can also use on-line dictionaries or kanji-databases to look-up unfamiliar kanji as you encounter them.
Jisho is a great resource of vocabulary and kanji when you are reading on-line. Just copy-paste the kanji or word into the search bar and it will tell you everything you need to know. http://jisho.org
Just as an example, the character 好 is pronounced hao in Chinese and means "good" but in Japanese the most common readings are "kou", and "su" as in 好き "suki", which means "(to) like".
You might see a correlation between "kou" and "hao", and conclude that they are related. And in fact they are because the reading "kou" comes from China (it has changed since then). These types of readings are called On'yomi (音読み). On the other hand "su" is a reading native to Japan. It has nothing to do with Chinese and it existed before Chinese character started to be adopted into Japanese. These types of readings are called Kun'yomi (訓読み).
Katakana is used for loan words. Words that have been borrowed from other languages, like English, French, or Brazilian, among others.
For eample, if you are looking for the bathroom, you could look for a sign with the japanese word, お手洗い which is written with hiragana and kanji or トイレ which is written in katakana because it is a loan word borrowed from English.
トイレ - toire Derived from English word "Toilet".
Here are a few more examples:
ハンバーガー - hanbāgā
テレビ - terebi
ハンカチ - hankachi
Please keep in mind that not all katakana words derive from English words, but once you start looking for them, it frequently makes it easier to understand these "foreign" words.