As you progress through Raid Mode you will spend time visiting the shop to purchase new weapons, attachments/mods, secondary items (grenades, etc.) and other items such as ammo bags for particular weapons and body armor. The weapons available for purchase change every time you visit the store, while new items such as more powerful body armor become available as you level up. You can either save up your BP (awarded to you for completing Raid missions and getting bonuses) for higher level items, or spend your BP willy-nilly. In either case, this should come naturally while you march to level 50 and 10,000 kills if you keep your armor up to date, keep your secondary items stocked and upgrade your weapons relatively frequently.