KWT My questions is where can I get the conversion rate from?
In order to get the potential exchange rates you will have to query Horizon for valid payment paths: Stellar API Reference - Payment Paths.
Depending on the type of payment (strict send/strict receive) there are two different functions you can use.
Each payment path will tell you the source amount that's gonna be used, the different exchanged assets and the amount in the destination asset. From it calculating the exchange rate is trivial.
The platform uses available offers to determine the potential paths it can take from asset A to asset B. If you want to provide liquidity for your issued assets you would need to always have an outstanding offer to guarantee the exchange. I haven't worked much with these so I don't know if you can limit who can execute those offers.