Google translate is very bad at dealing with this kind of expressions... Never trust it...
If I were really bad at something, for example English, I'd say:
Ich bin eine komplette/totale Niete in Englisch.
And if the topic were clear, I'd say:
Darin bin ich eine komplette/totale Niete.
Mein Englisch ist grottig/total übel.
(I think "suck" is colloquial, so you'd have to find a similar tone in German, as Heike did).
"Ich bin in ... sehr schlecht" :-) ... I don't really know, but sometimes there are no good translations for such things, I can't imagine how I would translate "I sucks at it" in my own (Czech) language..
Ich bin schlecht darin - I am bad at it. If you want to give it more oomph so to speak, substitute schlecht for other adjectives. There is no real verb equivalent to to suck in German, I think.