How strong of an influence would you say the decans have in your sun sign?
The same as in any other sign LOL
Or more particularly, the same influence as on any planet, other things being equal. It depends on what system you use to an extent but decans or faces are the lowest level of essential dignity so it adds a little if a planet is in it's own face, it detracts a little if it's in the face of Saturn or Mars (though this may be more than compensated if it has other essential dignity).
As a beginner, I'd tell you not to put very much weight, if any on it. Later on it's something that you can look at but for the moment the only essential dignities that you need bother about are rulership and exaltation and when you come to add others, take triplicity as your next. If you want to go beyond that at some point then consider the decans.
I do use them but they're still a relatively minor factor for me and the number of times they've played a vital part is effectively nil.