Eh, I'm finding Polish somewhat challenging to learn.
I can imagine ;) It's a difficult language.
How come my sentences never make sense? I mean, I thought 'cry' was 'płakac'. I though 'dress' was 'sukienka.'
You were right - "cry" is "płakać" and "dress" is "sukienka". But as XxxYyy already explained in Polish we inflect the nouns - their form changes slightly depending on the context.
"This dress is yellow" will be "Ta sukienka jest żółta".
But "What have you done with that yellow dress?" will be "Co zrobiłaś z tamtą żółtą
Can anybody give me some basic tips?
Well... Buy a Polish language course book where rules of Polish grammar are explained (noone will be able to explain it to you here, because that would be a lot of explaining, the grammar is rather complicated), maybe some computer course (interactive learning is more fun), watch Polish TV (if you don't have it, then watch stuff on the internet).