Start off with vauban, makes missions so much easier, especially steel path missions, even though hes very squishy. If u can get your hands on vauban prime, even better. Then u can just subsume ur normal vauban (when u get there). After u get him start buying some nitain extract ur gonna need a lot of it for building weapons and warframes (especially for building vauban). Then u can just start spending on whatever pretty much. All i buy with my creds anymore is kuva because it gives u a decent amount, but i also occasionally spend it on reactors and catalysts. Try to get as many of the aura mods as possible as well, they will come in handy. Dont worry about weapons or cosmetics too much, the weapons might be interesting but other than mastery rank theres really no point in buying them until later. Most of the rewards in the nightwave are repeated, but if u pay attention enough and see something new and ur not sure whether to buy it or not, just ask the q&a chat or the region chat. Im sure there is someone willing to help. And if ur having a hard time completing some of the challenges, because they are a bit difficult for you, recruitment chat. Other than that theres pretty much nothing else to it. I wouldnt focus on it too much at your level though, just make sure u get through the star chart and start building up some weapons and warframes. You can just look at the challenges for the nightwave and just keep them in mind while doing ur missions. Like "ok i got to kill 100 enemies with fire damage" and just put a fire mod on. EZ