How do you get started when learning a foreign language on your own?

I really want to learn Japanese in my own time as a side project. I'm sure lots of people learn foreign languages on their own. Any tips on where to get started?

start with the alphabets.

then learn some basic words, build some vocabulary.
then start learning grammar(making sentences etc) when you know a few dozen words.

google for learn japanese.

join lang-8.com. That will help with the writing/typing and you may find someone willing to skype with you and teach you japanese in exchange for you teaching them english...

I took it up on my own about a year ago.

The only thing you can do is dive right on in and start learning. There is going to be a lot you don't know at first, but try not to see it as daunting and just note down each new word you see (which will happen extremely frequently to begin with, but less as times goes by.)

First of all, learn kana. It's vital if you want to understand even basic Japanese.
Then, move onto kanji. A good place to get started with that is Heisig's Remembering the Kanji books, but there are lots of places online that can help guide you and give you advice on that. It all depends on the individual which learning method is best.
Look into buying a grammar dictionary/ grammar books - there are also websites where you can learn grammar for free, such as Tae Kim's Guide to Learning Japanese or Maggie-sensei.
Read in the language, listen to the language, watch films in the language - a lot. That is the only way you will learn it properly and have it stick, and be able to apply it in conversations without sounding unnatural.
Try finding a penpal (you can email, rather than send letters) who is a native speaker. It does wonders for your writing.

And be aware that it can be frustrating at times, but extremely rewarding.

Can you speak any other languages besides English? Because that can be a key factor in how well you pick Japanese.

I recommend you don't go head first into all the grammar/vocab/writing systems and just take it slowly. Spend about a month or so listening to Japanese music and watching Japanese film (with English subtitles), it will help tune your ear into Japanese and you may find that you will start to pick up words without any effort.

After this stage, get some phrasebooks and every week or so work through a section. Mix up your learning and try attempt the Kana/Alphabets too. Writing and reading is important with Japanese, but try to emphasise speaking and listening skills since that's the most natural and straightforward to learn a language.

At some point, try go to a Japanese class at a local college or uni, or find a tutor. They will guide you through the grammar and just fix any errors that will save you from embarrassment in the future. Plus they make good speaking partners.

Following this, you should then probably get into a system of learning 10 words, some Kanji (symbols) and a grammar rule every week or so. Take it slowly, practice little and often and you will make good progress. Don't rush or cram any of it, as the faster you learn something, the quicker it is to leave the brain.

Good luck!

watch anime and get a feel for the way they pronounce words, tho the japanesse in anime isnt polite its casual, you could also look at raws for manga and have a look at how they use the charicters