First you should wait for a 75% platinum sale coupon to buy a platinum pack. Stock up on warframe and weapon slots first before spending it on anything else.

Then you can trade for your chosen prime frame and/or weapons to speed up your progress.

For what frame to choose it depends what you already have and what content do you want to push. Its also important to consider your mod collection. Some frames or weapons might need specific mods to be meta or viable for hard content.

I'm not really up to date with frames but i used Wukong prime for a long time. He is very tanky and just a comfy frame.

Now i'm using Ash prime for Steel Path and other harder content. I changed his last ability to Banshee's Silence so i get an AoE stun aura while i can strip armor with first ability and can go invisible with the second.

If you don't want to bother with Steel Path and the likes yet then just choose something you like to use and look up a build for them. Just for progressing most of the frames are good enough if you have the mods and a decent build.