This is just the opinion of a student who's been at this for a couple of months. More than anything, you'll need to pick a course and start working on it. And yes, learning should be fun--that's like the absolute first rule. If it's boring, you're brain isn't working.
It sounds like your just starting out. If you haven't seen
AJATT yet, you should spend some time to look over the method. I don't agree with everything Khatz suggests, but found it to be very inspirational.
Get immersed. Listen to Japanese without understanding. This will make common vocabulary easier to learn. It will give you a head start on listening comprehension. Your accent will be better for listening. Beautiful women will stop you in the street and ask you out. Only one of these statements is false.
Now, vocabulary. Your goal is to get to the point where you don't need to translate to a scaffolding language. I've tried the method you suggest: "[Japanese] on one side of the paper and our native language on the other and learn that way." Simply put, it worked for Esperanto (I now use a monolingual dictionary), but was a miserable failure for Japanese. I think the more foreign the language, the better it is to skip single words for example sentences--and you don't get much more foreign than Japanese. (Don't worry though, millions of normal people speak it, so it's not like you have to be a genius or anything.)
You can also use example sentences to learn grammar patterns. Explanations are good if you understand them, but too heavy for everyday use. You need to memorize patterns, not explanations.
With flashcards, the game is won by volume, not complexity. Your goal is to learn
lots of very simple cards. As Einstein supposedly said: "as simple as possible, but no simpler." Eventually, you'll want to learn rare vocabulary and rare patterns, but you should limit cards to one point of complexity.
Starting out, you need simple patterns + common vocabulary, and I've found that Core 2000 delivers. There are only two downsides:
- A dozen hard sentences can easily take as much time as a hundred easy ones. Don't try to learn them all. Anki has a feature to automatically suspend cards that accumulate more than a set number of failures. I suspend anything that fails 6 or more times. Volume. Skip hard stuff--you can always come back later.
- There's adult-level business and political vocabulary, like「国会が再会した」= "The national assembly reconvened." It's not that bad (you'll learn those words eventually anyway), but I'd prefer even more of a focus on day-to-day and dictionary vocabulary. From what I've heard, KO2001 is even more business-oriented.
(That said, I love how「後に彼は総理大臣になりました」sounds like the last line of a bedtime story.)
Remember, the goal of RtK and Core 2000 should be to get you reading and listening to real material and understanding it--because that's when the real language acquisition really gets rolling. Strive to that and have fun and you'll do alright.