Do i pick a feat or ASI?

Im very much new to DnD and rushed into my first ever campain with some of my friends. I play a level 3 warlock with a plot relevant great old one patron. my ability scores are kind of everwhere because i didnt know what they each would mean to my characters stats. So i end up with really low dexterity. I currently have 10 AC and would like to increase it. We only play by PhB 5e if that makes any difference. Ive researched and found out that in lvl 4 i can get a feat called moderatly armored, where i can boost my dex too, but it wouldnt even help because i have 8, and that would only give me 9 and not enough to get 0. What do i do?