It depends on the plants and the lighting in the tank.
Generally though, very easy low light plants include java moss, java fern, anubias nana, anubias frezeri, and hornwort. All of these plants take their nutrients from the water column and do not need to be planted in the substrate--in fact if you bury the rhizome of the anubias and the java ferns, they don't do well at all. You can tie them to rocks, driftwood, or decor, and they will begin to attach to them with time. Java moss will as well. If you don't want to attach it to anything, simply put a plant anchor on the rhizome and let it sit on top of the substrate.
Hornwort is a nice bushy plant, very common, easy to find, and soaks up fish waste very well. You will have problems with this plant shedding its fronds if there isn't enough nutrients--but otherwise I like it a lot. You can allow it to float, or you can put a plant weight on it and let it sit on the substrate.
Since all you have in your tank is a single betta, there often isn't enough nutrients to make your plants really look nice and grow well. A good simple supplement to use if you just have a few non-rooting low light plants is Seachem's Flourish Comprehensive ( http://www.fosterandsmithaquatics.co...52&pcatid=4652
). You simply add a small amount with every water change to feed the plants.
You should know if you go to a pet store to look for plants that they often sell non-aquatic plants as if they were aquatic--if you decide to get adventurous at the store and experiment with other plants, stay away from bamboo, ribbon plant (lucky bamboo), brazilian sword (peace lily), mondo grass, purple waffle--and really anything that looks excessively colorful.