Any question answered...

Go to the Goethe-Institut.

https://www.goethe.de/ins/gb/en/sta/lon/kur.html

Don't do anything on-line. It leads to short-term gains which are quickly lost again and is not worth wasting your time on. Don't be put off by the cost. I've obviously never done of them, but I've heard only glowing reports from people who went there.

Persevere. German has more systematic grammar than English and the initial learning curve is much steeper. Many people give up and are left with the mistaken impression that German is much more difficult (as in the standard prejudice). That's not the case. Once you have got the basics together, it becomes easier, unlike English, which becomes harder to improve as you learn more.

Make sure to invest considerable time into pronunciation. Courses (even good ones) tend to aim at getting students to speak quickly, but it's really worth working on the basics. There are a number of sounds in German with which English native speakers typically struggle. The main one is the soft and gentle German ch as in "ich" (not as in "Bach", which is approximately as in 'Khan'). Most English native speakers end up pronouncing this either as k or sh. This can cause considerable confusion. Others include the long and very flat German "eh", which most English speakers pronounce as the diphthong in 'stay' (German has fewer diphthongs than English and all vowels are simple), the three "Umlaute" (which is a word that refers to the whole letters and not just the diareses on them) ä, ö, and ü (the German alphabet has 29 letters), as well as the German sch, even though this is very close to English sh. The German version tends to be fuller and more rounded and is a very important sound. Apart from these, the pronunciations are very close.

I have noticed it to be a considerable source of frustration for German language learners when they had already got quite far in the language and their inability to produce these sounds rendered their speech unintelligible, causing native German speakers to ask them to repeat what they had just said. The effect to a German is often one of sounds run together and not pronounced distinctly enough. It is therefore much better to practise pronunciation before suffering that kind of indignity. While it is hard for an adult to learn sounds they didn't learn in their youth, it is possible. Once you have those sounds, you will be much more confident and enjoy the experience far more.

You will need to learn how to perform 'glottal stops'. German uses these to structure itself and they are one of the most prominent features of the 'German accent', although most people don't know what it is that they hear there. They're present in English, too, although much less frequently.

Other than that, as zischendes.Reinigungsmittel said--make sure you find people who want to speak German, e.g. at a Stammtisch, go to German cultural events (not the Oktoberfest, as it is not a cultural event), read classic German literature (happy to provide suggestions). Listen a lot, speak a lot. Only practice works.

As has been noted before, Germans tend to learn English at school, and even though many of them end up sounding like German footballers called Jürgen, they are quick to switch to English when they notice you struggling. This is annoying, and you should be very clear with them that you don't want them to do that when it happens. They'll usually comply, as they thought they were helping you and didn't mean any offence. If you don't bring it up with them, you'll end up speaking English to them most of the time.

Anyway, feel free to ask any questions--it should be a lot of fun and of course I'd be happy to meet up about it, too.