I run 3x a week and go hiking/go to yoga fairly regularly so I have a decent selection of workout gear by this point. I also do not have a gym body, but I finally discovered that working out is way more pleasant when I’m not wearing a ratty old tee (which just tend to soak up sweat immediately and then stay damp against your skin the rest of the time anyway). Get some basics!
Old Navy is fine — I have a couple pairs of spandex running shorts and some racerback tanks from there. The tanks especially are worth checking out, since they often get the least amount of wear and tear. However, for shorts, I hate to say it but Lululemon makes REALLY good spandex shorts. My sister talked me into buying them during their annual sale (for ~$20) and they’re totally worth it. I can actually wear them for 2 or 3 summertime runs without washing them in between and they still don’t stink. They’ve also held up way better than the Old Navy ones in the wash.
For sports bras, I like the Victoria’s Secret zip-front ones. I don’t own any of their regular bras (they don’t fit me very well and the quality isn’t very good) but their sports bras are pretty great and come in bright colors and fun prints. And the zip-front makes getting out of a sweaty sports bra MUCH easier.