Is Thai hard to learn?

For English speakers, Thai grammar is very easy. Pronunciation usually requires a bit of effort at the start, especially if this is your first time learning a tonal language. Also, vocabulary is harder than when you learn a European language, as there is hardly any overlap between English and Thai, so you get no vocabulary "for free". Having said that, you can learn a subset of "survival" Thai in a relatively short time. And the extra effort needed for vocabulary and pronunciation is more than compensated by the time you DON'T have to spend learning verb tenses, cases, irregular plurals, agreement of nouns and adjectives etc. etc. (we have none of these in Thai).

What about Thai writing, is that hard?

It's a lot easier for westerners to learn Thai writing than it is for Thais to learn English! Thai writing is very different to English, and has some very strange features, but it is fairly regular so when you see a new word, you have a good chance of pronouncing it correctly (unlike English!).