Duolingo can be good, but it depends on the language. It's not great at teaching grammar, so I would try and get a grammar book alongside.
If you can get evening classes, thst will really help - many will start September, but a lot of providers have had funding slashed, so there's much less provision than there was. It also depends on the language.
There may be online classes - I'm learning a language via Zoom, and we have people from at least 2 other countries outside the UK, so timezones permitting, that might be an option.
Again it depends on the language, but some may have publishers which do books aimed at adult learners of different levels. There may also be magazines or online resources aimed at learners.
Otherwise, just getting newspapers, magazines, YouTube videos - anything in the target language. You're not going to understand it all at first, especially if it's aimed at native speakers, so don't be disheartened - but it will get you used to the sounds of it, to the look of the words - especially important if it's a non-Roman alphabet. Over time, you might pick out particular words then sentences. Documentaries can be easier - slower, more formal language than a fast-paced soap with arguments and slang terms.
Little and often every day if you can.
If you say what the language is, people may be able to point at specific resources- there could be a great variation depending on whether it's a European language, Asian, African, Chinese, but I bet there would be someone in MN who would know.