I'll go along with what's been said before: find your play style, pick a 'frame that works well with it, and enjoy playing the game while you farm the parts for other things.
I ran with Rhino for a long time early on, because I have no skills, and being able to take great piles of damage allowed me to survive to the end of missions better. Many other 'frames are supposedly very good (Trinity, Mesa, Equinox) but for whatever reason never got me excited. So others love them, and I own them, leveled them, but I don't take them out and wear them very often. One GentleTenno above mentioned his main is Hydroid, and he loves wearing it. Hydroid doesn't do it for me, and that difference between us is just fine.
Over time, as you play through Alerts and such, you will find that some missions can be made much easier if you own certain 'frames - Loki is a great example of this. Being invisible on Spy missions is a YUGE advantage. Do I put on Loki for pretty much anything else? Nope. But I rarely run a Spy as anything but Loki. I also don't run Spy missions unless I have to, because I like other mission types better. It's all about your preferences.
Nova's pretty squishy, so once I built her and maxed her out, I kept her in the barn for years. But recently (again, I have no skills) a kind Tenno explained how to build a Speed Nova, and now I enjoy running Defense missions with her - but as with Loki, I don't wear her for most other mission types.
So find your own groove and run with it, while you farm for parts and try out other weapons & 'frames. While you're doing this, ENJOY PLAYING THE GAME. It's the reason you're here, so don't neglect it.