Heavy Weight or Light weight training

20 Kg dumbbells can give you a good workout. (Notice workout and training are different, training consists of workouts which are designed to drive you towards a particular set of goals, whereas a good workout while being good for your health and wellness, may or may not help you reach your strength and conditioning related goals.)

Make the best use of those dumbbells, and superset them to increase the intensity. (Or maybe do a circuit workout without rest between exercises.) And when in future you can afford the gym, you can make more progress on the foundation you are gonna create with these adjustable dumbbells. (If you can find ways to constantly challenge your body, in moderation of course, then you can make decent gains with any approach.)

 

Heavy for strength, and Medium Light for Endurance. Heavy needs few reps(max 6) and around 4 sets, Medium Light more reps per set, e.g. 6 sets, 10 reps per set. Make sure your protein balance is in the positive. Some people, in extreme cases fast for up to 20 hours, work out using HIIT, and heavy explosive movements, using medium to heavy weights. Once the workout is done(no more than 30-45 minutes) take your protein. This schedule is insane, but scientifically it may be viable..Growth hormone plateaus at ~2,000% above baseline around 24 hours. The study stating this did not even consider working out. So I cant even imagine the GH levels after a workout, as well as a huge boost in testosterone. So now after you've worked out, then immediately take your protein (whey). Protein also increases GH, and maybe testosterone even further. The more weight you lose over time, and the more often you try to follow the workout, the more GH receptors will be available in later workout sessions.
GH metabolizes in the liver as IGF-1, which has links to cancer because of its main function which is to induce hyperplasia, or cell replication and or density. But if you're fasting, and taking in decent amounts of Antioxidant/Plant foods, in either case you shouldn't have to worry. IGF in my opinion is only dangerous when the body overall is unhealthy. After a 20 hour fast, I'm sure your body would be healthy enough to utilize IGF-1 without problem...

 

I train heavy on compound barbell movements, and light on isolation movments
heavy - 3 to 6 reps
light - 6 to 15 reps

 

Try both but what's important is that you should start a training routine and get hooked on it. When you see things you want to do differently in your routine, you can always change them. I'm on a bodyweight routine that focuses on strength and skills. I can't lift heavy but that's my choice.

All your plans are worthless if you don't act on them.

When you go to a gym you will want to ask questions. And when you do that you will meet some really incredible people. I will never forget the first time I went to my college gym. I made some amazing friends there who were so helpful and now I'm hooked on fitness.