Personally, I first began with flame meditation. Doing void on its own was way too difficult - having the flame as a visual distraction eased me into the ability to focus without random thoughts entering my head. After having done flame meditation many, many times - then I started doing void. The key here is consistency. Try doing it every day, even if only for 30 seconds. Watch yourself as you get distracted, and know you need to bring yourself back on course. Don't worry if wandering thoughts enter your head - it is certainly going to happen. But over time it gets easier and easier, where you can go longer and longer without wandering thoughts entering your head.
What I find also helps is to just focus on the sounds around you - whether being quiet in your room, or even during busy traffic while riding the bus - just focus on all the sounds coming in as a whole (and singling out any of the sounds). This basically works the same way as the flame meditation does, but instead of having a visual distraction from your thoughts, now it's an auditory distraction from your thoughts.
All in all - and I can't stress this enough - it simply just takes times, and practice, practice, practice. If you can't do 30 seconds once a day, even start with just once a week, and start ramping yourself up over time. Better do it than to not do it at all. And also important - have fun with it

Don't fret - if you can't do it for long, just laugh it off and know you'll try it again soon, and that it'll all get easier in time. Hope that helps!
HAIL SATAN!!!!