The easiest way is to travel to the marketplace map for your starter kit.
There you spawn at Beach 3 and go into the building at one of the Vault's that hang there.
Use the radial menu to select Reward Menu, then the store window will open.
Here you can click on Starter Kits and select the starter kit, by clicking on Purchase
at the bottom right you have "bought" it and now you can find it in the inventory of the Vault, where you can take it out and use it.

Have fun and success with it.