Pratica

The correction dipends on what you were meaning

If you want to say that you're going to eat all the torta you're wrong and you have to change tutto to tutta

If you want to tell us the torta means everything to you because it's so important to you than you're correct

voglio dire qualcosa importante

The easiest correction and the natural one is to insert "di" between "qualcosa" and "importante" but if you want somenthing more sophisticated you can put "che sia" in beetween too: the first is better when speaking while the second is better when you're writing. Along this way when you're writing or talking and you want to put an emphasis you can put in between only "davvero" meaning "very".

voglio essere certo prima decido

The easiest correction and the natural one is to change "prima decido" in "prima di decidere" but if you want somenthing more sophisticated you can put "che" beetween "prima" and "decido" and if you want to achieve the better and more precise meaning you can go with "prima che decida" meaning you have a doubt that you will decide.

non credo lui proprio lo dice

The easiest correction and the natural one is to change the phrase into "non credo proprio che lui lo dica" meaning you have a doubt he's going to say/tell it. If you want to go with some more intriguing correction you can change this way: "non credo proprio che lui lo dirà" meaning you have a doubt he's going to say/tell it but somehow you think he will not.

non sono sicuro di come farlo

That's perfect.

ho un paio di due

It means nothing! I can't guess what you mean because word by word it sounds like I have a couple of two so or you have a couple or you have two meaning the same thing that's in Italian "Ne ho un paio" or "Ne ho due"

questo leone ha molto coraggio

That's perfect. But as in English it's truly uncommon to use this lion instead of that lion also in Italian even if the prase is correct you probably will have "quel leone" more easily than "questo leone"

Pluto non è più un pianeta

Pluto in Italian is Plutone. Changing it you're correct.

voglio un certo tipo di vestito

That's correct but you should specify which one unless you have an incomplete phrase that gramatically stands but it doesn't stand logically.

Pratica

Pratica