In this page you can find a walkthrough for the prologue in Uncharted: Lost Legacy. It is a very short stage during which you only have to go through a marketplace and sneak into a truck. During this stage you can unlock first two secrets and the level is made in a such way that you really won't be able to miss them.
When you begin the level you must start following a girl called Meenu. During your walk through the marketplace you don't have to look for additional objects to collect because this part is very linear. You only have to stop next to the place for taking a photo (1/1) and press Triangle (you can check it later by selecting your phone from the inventory).
Soon, Chloe finds out that Meenu stole her artifact. Luckily, you don't have to react to this in any particular way. When the girl promises to give the item back you must press Triangle to "officially" get the treasure (1/1 - Ganesh Figurine).
Continue following the linear path. Shortly after leaving the marketplace you will get closer to a palace with a few military trucks parked near it. Meenu will automatically start distracting one of the guards. As Chloe, you must quickly get closer to the back of the truck, presented in the picture, and climb it. Don't try to explore the area because the truck will leave which fails the mission. Get into the truck and you will complete the prologue.