Three things you need.
An algae scraper, a magnetic one or one like a nylon scouring pad.
A syphon/gravel vac gadget.
A big clean bucket.
Now...What you do... Each week.
Turn off the power to the heater and filter, you dont want those running when they are out of the water.
Run your algae cleaner around the glass. This is just a cosmetic thing. The glass can be completly covered with algae and it wont harm the fish, just you cant see them any more, and it looks ugly. It wont harm the fish though.
Use the gravel vac to suck as much crud as you can from the gravel while removing about 1/4 of the water. The actual amount isn't critical, and the minimum you need to change depends on whats in YOUR tank, but 25% is a good average number.
If the fitler is blocked and the water flow has slowed down then rinse the sponge out in the old tank water you just removed. Dont worry about getting it spotlessly clean, just get most of the gunge out of it.
You can take out individual rocks and ornaments and give them a wipe down too if you like, but again it's only a cosmetic thing.
Top up your tank with de-chlorinated tap water at about the same temp as the tank water.
Remember to turn on the power again.
This routine works with any size tank, but as they get bigger the bucket thing becomes a pain and people use things like the Python syphons or garden hoses. But the idea is the same.
Ian