Alan, I'd recommend Duolingo's Greek course (https://www.duolingo.com/
). It's online, free, assumes no prior knowledge of the alphabet, has a very helpful community and will help you start learning the grammar once you move through the lessons a bit. It takes a little while to get going, but stick with it, it's incredibly useful. I reckon that's one of the best starting points you could take.
Once you've learned the alphabet, there are a number of good courses on Memrise (https://www.memrise.com
) which will help you to build your vocabulary.
Also once you've you've got a bit of grammar and vocabulary under your belt, Clozemaster (https://www.clozemaster.com
) can help with comprehension and context. I'm also a big fan of reading children's books (I'm currently having a lot of fun working my way through the Mr Men series in Greek).
Alf already recommended Language Transfer, and I'd second that. Definitely worth listening to.
To give a bit of context, I've been learning Greek for about 18 months, in London. I'm nowhere near fluent, but can communicate in Greek reasonably well and have a vocab of around 10,000 words. I don't speak Greek well enough to talk politics, or get a phone connected, or anything too technical, but enough I can have basic conversations and can understand anything written that isn't too complex. Understanding spoken Greek which is spoken at Greek speed is still a real struggle for me, but it'll come with time, especially once we're living there.
I'd also agree with others who say that most Greeks make plenty of errors; I've heard that from countless native speakers and even at my relatively basic level I often come across spelling and grammar mistakes on shop signage, in shopping catalogues and so on. Almost every Cretan I've met has been very supportive of my efforts to try and learn the language though, and I think you'll get a lot of respect for trying. Good luck!