Hello EnderCrypt!
Mephansteras made a very good recommendation there. Putting something like "English isn't my first language, so please forgive me if my English is not perfect." into your signature can help you and the rest of the community avoid unnecessary confusion. As I mentioned before, not everyone reads everything, so this thread may be unknown to a lot of people.
As for general rules, I advice that you check up on the basics, as they are a very easy way to improve the impression of your posts. The personal pronoun, first person singular ("I") is always capitalized in English. It is odd, I know, but that's a simple rule, and missing that one sticks out quite harshly. If the copula "be" or auxillary verbs like "will" are shortened, don't forget the apostrophe ("'"): I am -> I'm ; you will -> you'll
I know this may sound tedious, but these are the words and word combinations you will be using most often, so getting them right will have a major effect on things. And always capitalize nation names and adjectives describing nationality: I am Swedish; I am German.
In general, I want to make three recommendations:
Read a lot: Reading the posts of fellow forumites gives you an impression of the accepted style of English here on the forums. Language is a very dynamic thing, and there are lots of various sociolects. What is alright for one group of people may be intolerable for others. In addition, reading a language gives you a feeling for the flow, helping you to subconsciously structure your sentences. Beware, however, of threads which are games or jokingly working with the language, as they may deliberately deviate from the norm.
Personally, I recommend the "What's going on in your Fortress/Adventure", and "Note to Urist" threads, as they sometimes include small stories, but there are many, many more enjoyable threads especially in the Dwarf Mode and Adventure Mode sub forums.
Use a dictionary: I know, this may sound like a lot of work, but in the end, it usually pays off as it allows you to express yourself efficiently. And over time, you will learn the words you look up regularly, meaning that you will not have to use the dictionary that often. I recommend that you stay clear of translation machines as they may produce sentences that are seriously mutilated or fail to convey the original meaning. It is safer to simply check individual words in a bilingual, bidirectional dictionary.
Paraphrase: Alternatively, you can try to paraphrase what you are trying to say. That is, you write two sentences instead of one, using the additional space to describe the concept for which the proper word is lacking. People will not be mad at you if your posts are a bit longer because of that, so don't worry about that.
I apologize if I sound too much like a teacher, but I felt the need to give that advice. I hope my recommendations are somewhat helpful.
Yours,
Deathworks