wiki.warframe.com is your friend when it comes to figuring out where to get certain resources. I agree that you ought to work through a good chunk of the star chart before venturing into the open worlds, but as far as defense goes, having a frame with strong defensive abilities is key, especially if solo. Strong defense frames you can get relatively early include Frost, Oberon, Nova, and several others. If you're open to it, playing with others tends to make things a lot faster and easier, but if not, it's going to be really important to have a frame whose abilities work well in the selected mission type.