You should really try to find a comprehensive system, like an online class or textbook. You can't just learn the writing system; you will have nothing to write. Learning a language means learning grammar and vocabulary through reading, writing, speaking, and listening. Think of Hiragana and Katakana as the way to spell words. There is no point moving on to Kanji if you don't have any words to spell. Start learning basic conversations: greetings and introductions. If you don't have a site or textbook, you might try Japanesepod101.com It is free to start, though you have to pay later. But it has a lot of great resources on grammar and conversations.

Well you'll have to find a learning method that works for you, that's how any language goes. Kanji is something you'll work on for a long time alongside whatever else you're learning. You'll need to decide on methods to study grammar and vocab. Also find ways to practice all four language skills: reading, writing, listening and speaking. I would personally suggest either finding a class or a textbook that you are comfortable with and set a schedule for yourself, if you're serious about it. If you don't want to invest too much I'm sure you can find free beginner grammar lessons online somewhere, but most free material won't get you past a beginner or low-intermediate level. Lately I've just been using the JLPT as a guide in a way. You could pick out the easiest level N5 and just say that you want to feel confident in the skills you'd need to pass the N5 in a year or 6 months or whatever. Then start the next level. It's hard for me to tell you how to begin studying on your own, I took college courses for Japanese and I personally believe that's the best option. Easy access to a community, support, and a framework to structure your study around. And once you're done you know HOW to study. If you really commit yourself to using communities like italki and lang-8 you might be able to make it work though. There are great resources online but I've always thought of them as supplemental.

after katakana and hiragana i started learning Japanese words which exist in katakana only, like テレビ and ホテル :)

Okay thanks everyone, but I will probably not take a class since I am only learning Japanese on the side. I already have a full load at university. I guess the next thing to do is learn phrases and how to write and read them.