If you are in economy you will be able to upgrade to premium economy only using Avios (you can only upgrade one cabin). There also needs to be redemption availability in the next cabin on your flight, so with that in mind you are best checking this using the redemption checker on BA.com when booking your flight to ensure you can upgrade.

If there is availability and you want to be 100% certain you will be able to upgrade, you may be best to call BA to book as they can process the ticket and the upgrade at the same time - there will be a cost for this service however.

Regarding the cost, use the tool on the BA site. The cost in Avios to upgrade will be the cost of an Economy redemption, subtracted from the cost of a Premium Economy redemption. (e.g. if its 10k for a Y redemption and 15k for PE, the cost of the upgrade is 5k).