I tried that version of DOTA they have now. I couldn't seem to kill anyone, and I couldn't figure out how to do the "combined items" thing. Also I died a lot. Does anyone have any tips on how to play it?

I could be wrong but I think some of the standalone DOTA games include tutorials, check out League of Legends or Heroes of Newerth.

" I tried that version of DOTA they have now. I couldn't seem to kill anyone, and I couldn't figure out how to do the "combined items" thing. Also I died a lot. Does anyone have any tips on how to play it? "

So did they even add a tutorial for their own game?  Are they just expecting that everyone already know how the mechanics work?  Some of us know it already, but it is really stupid of them to not even have an option to just go through it with you before you begin at the very least.
 
Just play the tutorial of League of Legends, and get an idea of how this kind of game generally works.  Then in that new version of DOTA, just try to take your time reading the recipes and figuring out which will benefit your selected hero the most.  Just experiment for now and ignore any rude people noticing that you are taking said time (there will always be one that will complain).

I'm actually talking about the one in Starcraft II. There wasn't a tutorial that I saw, but I also didn't spend much time in the base thing.

Are you sure you played DOTA or Storm of the imperial Sanctum?

If you are new to the Dota genre, I suggest you keep playing.
 
Dota games have a high learning curve.
Read guides and watch replays.
 
Once you thing you're ready to go hardcore, you can try out the competitive stand-alone Dota game called Heroes of Newerth.