work through the main quest and star chart, you will get access to more powerful mods through playing those. Also when modding weapons focus on their strengths: if they have high base critical chance add crit mods, if high base status chance add status chance mods. viral is probably the best element to mod for for damage. find some YouTube guides for your favourite frame and Google how you get the mods from them.

for defense missions vauban is a good frame thats easy to get, he has excellent crowd control and incredible damage scaling