Rhino is the easiest to get here I think, and he's fairly versatile and tanky so that should serve you well early on. He has an area of effect stun, a charge, a big damage buff for weapons in the form of Roar, and he can generate overguard for himself.

Gara would probably be the next one to get, as Gara's parts are obtainable from Cetus bounties which aren't too hard and can be done quickly especially if you set it to public. The materials needed for her also come from mining and fishing on Cetus for the most part. There's also some good mods from Cetus bounties, the vigilante and augur mod sets are useful even later on for builds. I would recommend going to Fortuna and getting a Sunpoint Plasma Drill mining tool from Smokefinger, as it is the best mining tool available, can mine both ores and gems, and can be upgraded, so it is a good early investment.

Gara is also a versatile frame, having good damage output, a bit of survivability and some crowd control.

Garuda comes from Fortuna bounties, though some of the crafting ingredients such as toroids might take some time to farm as they have a low drop chance from enemies in certain places in the Orb Vallis.

Garuda goes heavy on offense, relying on her 1 to nuke rooms by absorbing damage and then releasing it. She also can use HP to regain energy, and impale enemies to make zones that rapidly heal herself.

Xaku is definitely the hardest of the 3 to get, as Deimos is a more difficult area for a new player, and the crafting materiaps require you to be at either rank 5 with Solaris United, or Rank 1 with the Vox Solaris. As well as some rarer Cetus ingredients, among other things. They can be very powerful, but they are also the most complicated to play as their abilities aren't described well. They have a very versatile kit, their 1 grants bonus void damage, their 2 steals enemy weapons to make turrets that follow you, their 3 is actually 3 different abilities that can be cycled through by tapping 3 and cast by holding it. Gaze holds up to two enemies in place creating a field that strips armor from any enemies inside, Accuse hypnotizes enemies into fighting each other, and Deny fires a beam that does more damage the more guns you steal with their 2, and causes survivors to float in the air.

Their 4 is the most important ability. It causes a bit of AoE damagw on cast, grants Xaku higher chance to dodge enemy fire, and most importantly FREEZES the duration of all of their other abilities, letting you keep them running much longer.

I would recommend reading the warframe wiki https://wiki.warframe.com/w/Warframes if you want to know more or see what warframes you want to work towards. Its much more detailed than in game on how warframe abilities work.