Welcome to the site!!! The best advice I can give is to read as much information as you can before you put money into trying things out, it can get expensive quick. The need for wanting more or something better always takes over this hobby, hence my name lol...
The best way to turn your tank over to a planted aquarium would be to find a new home for the goldfish. Also, some plecos can be plant eaters - depending what species they are, and get to big for 37g. Some species are mainly veggie eaters, meat eaters, or both. It is very important that you also have a piece of driftwood in the tank for your pleco as some need this in their diet as well. Do you know what kind of pleco your looking to get?
Also, your substrate is VERY important. You can use what you have, but will have to add root tabs to provide nutriets to stem plants unless you change your substrate over. Which you could research for weeks all the different substrates people are using. Check out the sticky in the substrate section for more information on that.
As the others have pointed out, certain plants are almost guarenteed to thrive in most conditions. Anubias, java fern, mosses will slowly grow in a low light setup that you currently have, but these need to be tied to something. So if you dont have a lot of rocks or driftwood, it will look awkward with not having any hieght of plants in your 37g tank. So you will want some stem plants to grow in the background of the tank to give a more complete look.
Depending on what fish/inverts you do purchase some swim at different hieght levels so it is good to have your plants at different hieght levels too
Recommendations on choosing someone to buy stuff from - every person has a rating next to their name (whatever number out of /100%). You can click on that and it will show you some information about what that person has bought/sold and if people were happy or not with what they got. I tend to lean toward people that have sold more because usually its better quality plants and packaged better, but some people need to start selling at some point so don't always discount them.
The main question is what you want to acheive, and how much are you willing to spend to get there. Ask your self this, and research tank journals to see other peoples setups and what you like.
If you have any other questions, I am always willing to help the best I can so PM me. Good Luck with your journey!