If you are just looking to get a couple of plants and gradually add more, start with an anubias and a java fern. They are about as bullet proof as you can get. They are rhyzome plants, so they just attach to a piece of rock or driftwood. I attach mine to plant weights so I can move them about, or remove them if needed.
They don't need high lighting, I literally have java fern in an unlit aquarium. They don't need much in the way of nutrients, an all in one fertiliser like seachem flourish with your weekly water change will do.
As the plants grow you can cut the rhyzome and get new plants. The java fern will produce plantlets that you can separate from the parent plant and get more plants that way too. You can gradually plant a whole aquarium from a single java fern if you wish. Anubias are a little slow growing, so if you want more of these it's quicker to buy more though.
Other easy plants to consider are amazon swords, cryptocoryne, and cambomba. These are rooting plants. Java moss can be tied to your aquascape too, similar to rhyzome plants.
This is episode 1 of a good series of videos setting up a low tech planted aquarium.
This is a good video suggesting some easy plants.