An adult language learner should start all the languages the same way - learn orthography and pronunciation at the same time.
Definitely, I would start speaking and listening and... repeat, repeat and repeat.
Try to study key tenses to speak about basic topics, for example:
- Introduce yourself:
Tengo _____ años
Soy de ___
Vivo en ____
Estudio español porque ____
After that, I'd start with daily routines, descriptions, hobbies, etc.
If you want to study grammar the first step is present form.
I use this system with my Spanish students and works pretty good.
Studying: I recommend you flashcards, some platform like anki or quizlet are good. I love them.
Listening: I recommend you www.podcastfromspain.com
Speaking: you can try to find some language partners or you can try to book some conversational lesson with community tutors or professional tutor here, on Italki.
I started with www.duolingo.com ,it is free,fun,and helpful. I also learn from YouTube videos (señor Jordan ) and i downloaded some apps too. I try to listen to spanish songs everyday.
Pimsleur or Michelle Thomas.