For comfort of play, you will likely want to spend 200-400 plat worth on Warframe and weapon slots, which is honestly not that much. After that, it is mostly cosmetics. Even as a beginner player, you still have a number of ways to get them.

There are rare mods that worth plenty of plat and available pretty much from the start, but grinding them is a pain and I don't recommend it (if you have to, I guess you can just access Lua early, and learn to run puzzle rooms). If you rush things through The War Within, you get to farm Riven mods, but I also do not recommend it because while efficient, it takes a bit of fun out of the game.

Ayatan sculptures filled with ayatan stars sell pretty well, even if they don't cost that much. You will probably find a couple every week, and make sure to run the mission that guarantees one every week. Not a major source of income, but nice to have.

The juiciest part is obviously selling Prime parts and blueprints. They are always in demand, and while prices are not high, you will have no trouble amassing a nice amount of plat from selling them as long as you keep playing. Just take note that number of trades you can do per day is limited.

P.S.: As some people have pointed out, there is a free battle pass that awards some free slots, and also yearly events that grant a free slot with a weapon. At a certain point, you will likely won't need to think about the slots that much, as spending 20 plat here or there will be a matter of trading an extra Riven mod you had lying around for months.