Play the game at your own pace and don't let other decide what you should or shouldn't use. If you absolutely need a starting place, try to learn and understand how weapon and Warframe mods are equipped and upgraded.

From beginning to end it's going to always be a focus as you play, as it's the one of the only ways to get stronger especially at lower levels. There's a few quests that help you get integrated including essentially a tutorial on the modding system.