Save your Credits for Better Weapons
There are two different currencies in Aliens: Fireteam Elite, these are your regular credits which you can redeem for perks, weapons, and the various attachments for them and Scrip. Scrip can be used for skins, gun decals, cosmetic items, and Challenge Cards. It will take you several missions before you have earned enough credits to purchase a gun outright from the requisition store. When you consider that your progression is individual to each weapon, it is recommended that you make saving up the required credits for a better weapon a priority. If you are not sure which weapon to go for then save up for the M42A2 Scout Rifle which is a heavily modified version of the standard M41A2 Pulse Rifle and makes the Xenomorph runners that are fall out of vents, climb up walls, and run across pipes a lot easier to kill.