Welcome to the hobby and OUR forum!
For a new tank, add in the declorinated water right away, run the filter+aerator and let it stand for a week before adding the fish.
Get only a few fish first to get the cycle going.

Small tetras and platies get along fine. There are many many diff. kinds of tetras, you could mix most of them. Just make sure you get at least six of each kind so they get to socialize (gee, fish socialize?). If you plan on adding some plants, avoid the red-eye tetra (that's their meal).

To clean slime/algae off the glass and bottom, get a Chinese Algae Eater (normal, goldenred or albino)(that's after one month later so there will be food for them to eat).

20% water change weekly is a must.
Someone, please advise on the red-tail shark!!!!
-- Zaini