The best way to gain happiness, as all the religions speak, is the service to human beings. If you servce your fellow people, you will automatically derive a lot of happiness out of it.
When you love any one whole heartedly and kindful to them, there will be a natural reward for you in form of true joy. The more you love, the more your heart blossoms. The larger it blossoms, the more happiness it can hold. But by just saying 'I love entire world' or keeping your love without expressing is no use to anybody. You should express your love through dees of kindness and service, which pays you back a lot of joy. By serving people, you will get a lot of gratitude for them and it fills your heart with a lot of satisfaction and happiness.
When Prahlada confronts with his father, Hiranya Kashyap and says, 'God is everywhere', the egoist father furiously orders him to show in a pillar. As Prahlada prays to God, from the pillar the Lord Vishnu comes out and kills Hiranya Kashyap. This story teaches us that God is everywhere. He is in you, in me and in every living being. So serving every human being as much as you can is nothing but serving God, where you can derive yourself a lot of happiness that you also made God happy. And you can see the help you provided to others also working well.
When you want to serve people, you also should help those who are really in need and require help. For example you can not buy costly gifts and give it to the beggers, thinking that they do not have it. Beggars need good food, clothes and shelter. If you can help them by providing any one of these, they definitely feel happy. What you give, gains real value only when you give it o those who can never get those. If you give a toy to a poor child, he really enjoys it and you can also have the pleasure of the child's enjoyment out of your gift.
We always be ready to serve people. Suppose if you are going to office and you find a person injured on the road in an accident, call to you office and get permission to help the poor guy, who is need of treatment. Call to ambulance and ask help of others on the road to rush him to nearest hospital. Do not think that I have to go to office, there are so many people on the road, who can help. If everyone thinks so, there may be a life threat to the injured, so do not neglect your opportunity to help.
Service to people is service to God. Thinking of serving people itself is a thought of divine. But these divine things, as opportunities will not come whenever you want. You have to get those. Whenever you get you should not miss them.