If you buy yourself a gravel siphon, you can suck up the left-over food and poo that's dropped onto the gravel.
The amount of water you choose to replace is up to you - you could do 20-50%. With only 2 fish, one (9-litre) bucket should be ok, IMO. Do it regularly once a week. Get yourself a couple of buckets to use - keep them solely for use with your aquarium.
Fill one bucket with fresh water and add the amount of water conditioner to treat the amount of water in the bucket. (If you want to change more than one bucket, it's easier if you have enough buckets to fill up beforehand, otherwise you have to go back and refill it)
Turn your filter off. Use the siphon to siphon water from the tank, stirring up the gravel and getting the muck up so it goes up the siphon into the other bucket.
Once you've siphoned out some water, use a cloth (again, get a new one, only use it in your aquarium, never put any soap or anything on it) or get a sponge-on-a-stick type cleaner to give the interior walls of your tank a wipe over if they need it.
If you need to clean your filter (depending on the type of filter and assuming you've got pads), rinse your filter pads in the water you've taken out of the tank. You don't need to do this every week - do it when your pads look mucky. After a while when they are really clogged and mucky, change one for fresh - don't change all of your pads at the same time.
Then you can slowly tip your fresh, treated water into the tank.
Turn your filter back on. (I wrote on a piece of paper "TURN THE FILTER BACK ON" which I take out of the aquarium cabinet and put on the table when I'm cleaning the tanks so that I don't forget... but I'm a bit senile, you might not need prompting
Edited to add: Is this a completely new tank? You might need to do more frequent water changes if the tank is cycling - see the sticky thread at the top of the forum about cycling a new tank. After the tank is cycled, once a week is fine.