You'll need a little more detail than that to get a useful answer, unfortunately.
Are you a developer and/or someone with access to custom development? If so, there are great options like Stripe available. You can use PayPal's API.
If not, you may want to look into SaaS offerings that handle the UX for you.
Also, is this a native mobile app, or a web-based mobile app?
Do you need subscription payments or one-off payments?
Taking people's money online is a complicated, risky process that needs to be extremely well-considered. Otherwise you risk big problems: unhappy clients, security vulnerabilities, poor usability, and so on.
I would recommend doing a good deal more research on your own, then coming back to this question with far more detail.
1. To create mobile payments you should start with peer to peer payments and let people pay anyone from their contact list.
2. Piggyback on messaging apps for faster adoption.
3. You will have to figure out how people will send money into mobile wallet and how they can take it out from mobile wallet. This has to be done within the ambit of country regulations.
4. Once money is in wallet, let people send and receive money via your mobile wallet.
5. Create a new wallet for anyone who has not created a wallet but has received money
6. For mass adoption you will have to focus on interoperatibility, meaning:
6a. Ease of transacting between mobile wallets provided by different wallet operators (will potentially address all payment scenarios - peer to peer, customer to merchant, and business to business payments)
6b. Ease of transacting between mobile wallets and bank account (will ease cash-in and cash-out)
6c. Better dispute resolution processes
6d. Better fraud and risk mitigation policies