I have one tank that sometimes develops algae on the wall facing the window. (Fluval Spec V tank is on windowsill) When I do a large water change, I easily scrape it off with a credit card. Otherwise, I fully agree with the recommendations of the above posters!!!
Right now I have Java fern, java moss, anubias, water sprite, jungle Val's and corkscrew Val's. I haven't totally killed them lol and I'm a plant noob. I'll probably be getting more anubias and duckweed and maybe crypts.
Check to see if your local water supply has excess phosphorus. I know my water has a lot of it (I use tap water in my tanks at college and phosphorus runoff is crazy). You can call and check your local water waste treatment plant.
If you have too much phosphorus, I am sure you can get some chemical thing, or you can just get a buncha plants. Also, a marimo moss ball will help. Any competition (moss ball would be better than plants, but plants are really great, too) will cause the algae to grow less rapidly.
You can fit quite a few plants in a 5 gallon. Some people make tiny planted jars with a variety of plants. Bettas actually like plenty of cover so you could fit quite a few, and IMO, live plants are fun because, like the fish, you have to learn to care for them and the different types. (I guess I might be weird because I LOVE learning about both fish and live plants.)
There are actual planted plants that you bury in the substrate, stem plants that you tie to decorations (like anubias and java fern - these will rot if you bury them), and floating plants like hornwort, water sprite, and duckweed. Floating plants and stem plants generally grow faster and planted plants grow slower, but a variety of plants is best. I love anubias plants, just the look of them, as well as my water sprite which grows fast. I also think that java moss is neat looking! And duckweed, but I hear it makes a big mess and is hard to remove once its in your tank.
Also, live plants will grow and propagate, or make new plants (they way they propagate differs between plants), so if you have enough live thriving plants in one tank, you can transfer them over to any new tanks. They can literally be a one time expense (as long as they don't die).