please help . i dont know how to record songs from my SP-303 to my computer.
I have a Midi cable but not sure what else I need .
I have read i might need a soundcard, but am not sure, so i dont wanna buy one , if i dont need it.

HELP!

Midi cables send midi messages..not audio

You need an audio interface. An audio interface just takes the place of your computers soundcard, usually bypassing it completely. Its external btw.
This isn't something you want to just to buy without doing some research first. Try to get something with at least TWO 1/4" audio inputs and TWO outputs. Your going to want to spend a good amount of money on your interface.

Judging by this question you probably don't have studio monitors either. You're definitely going to want to invest in some of those also. Do research on these before you buy these also. You want to spend the most money on the monitors and interface. Monitors are usually sold not in pairs. Lots of people first starting out have KRK Rokits or M-Audio Bx5s. These are good options, nothing wrong with them really.
You don't want to skimp out on monitors they are the one of the most important parts to your setup, if not the most important.

Making beats is an expensive hobby, you should learn that NOW.

Lastly your gonna want cables to connect everything together
Sp303>Audio interface>Studio monitors.

Depending on what kind of equipment you get, you might need different cables.

I spent around 400-500 on just my monitors and interface and i did my research to try to get the best options for the money.

If you think about how much your spending, you need to realize that your probably gonna have this equipment for years so you might as well get the best options for your price range.
If you aren't ready to spend this kind of money...save up. if you have to wait a little longer to save up, do it. You'll be kicking yourself later if you bought the cheaper options just cause you were impatient or being just cheap.

Get an RCA to 1/8" mini cable and hook the output on the SP to the Mic input on your computer's sound card.
Download a program like Audacity or Reaper to record with.
DONE!

Getting an audio interface and some better software, like Pro Tools will have a better quality and give you more options.

Yes what MattCoops said is true- most computers have 1/8" inputs- though some older are mono. You will need stereo RCA male to go from 303 output to stereo 1/8" male to go to laptop or desktop mic input.
Pic at bottom.

For a free full featured software you can use Audacity or I like to use Reaper.
http://www.reaper.fm/

You can also get USB based audio interfaces for $9 to $50 that work fine. You can spend more and more and get more and more features and sound quality but I have the little Behringer 302USB and it is acceptable. Not as quiet as some but just depends on what you want to spend. My personal main interface is a Mackie 820i mixer and I have expensive preamps etc. but often do not use them. I have recorded plenty of mixes right into my laptop 1/8" input from SP and came out fine.

The Behringer or Syba products on this page would do the trick.
http://www.amazon.com/b?node=11973691
Some would require the cable below while others would just need regular stereo RCA to stereo RCA also shown below.

What model computer do you have? We could find out easily if your mic input is mono or stereo.

Only problem sometimes is buzzing from ground loops. I won't go into that but if you can run your laptop on battery long enough to record what you want you could get around that issue cheaply (free)

C