SO the money should come from them, like any service, they pay you for parts and service. Warranty will cover individual parts, however, as a service im not sure what your legal obligation would be to offer.
That said, if you break the parts by doing a bad job, your way, way out of pocket as youll have to refund the customer or replace the parts yourself.
In terms of warranty you may need to find out what length you will have to give them. It may only be 12months, it may be no warranty what so ever, buit you need to check that. Other than that you don't actually need much money yourself for it. Maybe just for tools, esd bands, screwdrivers etc.
(about charging them after, if you do do this, please never ever do that. If they change theyre mind and dont want it after everythings paid for your out of pocket
Always get your costs etc up front. To make it more favourable for the customer, charge them for parts before and for your service after. This may help with the fact your young and as they will see it not very experianced.)