When will we be able to put our collected minerals etc to use and start synthesizing ammo and stuff?
As has been stated you can already do this once you have enough materials.
In the same location as you can see the number of materials you've collected (in one of the right-hand panes) you can selected the 'synthesis' tab.
Available options are highlighted in orange and in either case you can select any of the options to see what materials are required and how many of that synthesis you can currently carry out (if any).
There are three levels to each possible synthesis type a basic, normal, and enhanced variant (I forget the exact names). Each has different material requirements and can be invoked independently as available and desired (so you can elect for basic ammo even if you could have the advanced one). The more advanced options provide additional benefits but require materials that are rare.
Available synthesis options are adjusted by whether you are in the SRV or in your ship, and the ones possible are shunted to the top.
The SRV allows SRV repair, ammo refile and refuel. This should allow you to carry on more or less as long as you like on the surface provided you can find the resources.
The ship options are more varied and include things like boosting your FSD range and ammo refills.