Start with a business plan.
At a minimum, you should be familiar with the currency exchange business. 
Allocate your funds. How much money you are going to operate at the initial stage.
Determine which payment systems you are going to work with. Register in these systems, create accounts.
Choose the best exchange office script. If you do not have enough features, contact the developer.
Come up with a domain name. It is better that the name is easy to remember and it corresponds to your line of business.
Choose a hosting provider. This is the home of your exchange. It should be reliable and stable in operation.

What to earn on
1) On the spread - the difference between the selling rate and the buying rate. 
For example, you can sell 1 Yandex-money for 68 rubles, and buy for 65 rubles. 
Your earnings from the turnover of 1 Yandex-money (from its purchase and sale) will be 3 rubles.
2) Your own commission
You can profit from the commission charged for exchange operations. 
In this case, the spread is usually zero, and the exchange rate is as close as possible to the current rate set by the banks of the countries.

Funds to start with
It all depends on your ambitions and regional coverage. 
You can start with 500 USD. But of course, the more the better.
The optimal amount is 3000-5000 USD. The availability of such funds will allow you to exchange large amounts of money and quickly gain a base of regular customers.

Exchange rate
You can set the exchange rate you want.
Low exchange rates = more clients, but less earnings.
High exchange rates = more earnings, but fewer clients.
A customer will almost always look for the most favorable exchange rate.
Therefore, it is optimal to set the rate taking into account the current rates at competing exchangers.

Expected amounts
There are clients who deposit or withdraw 10-100 USD. 
There are those who operate with amounts up to 10 000 USD and more.
It is most pleasant to work with "middle class" clients (200-1000 USD). 
Usually such amounts are always available and earnings on them are quite substantial. 
The biggest problems are caused by "small" clients operating with small amounts. 
Usually they are beginners. They are distrustful of your exchanger, your business and you personally.
Earnings on small amounts are small and do not pay back the time spent. 
Therefore, it is optimal to set a minimum amount of exchange. For example, from 10 to 50 USD.
Limitations will save time and effort, but do not overdo it. 
One of your main tasks is to form a circle of regular customers around you, trusting your site and you personally.