The viewing panel you can clean with a regular aquarium algae sponge. Don't use a sponge from your sink or from CVS or something, because a lot of them have anti-microbial agents, and that'll harm your aquarium. So use a regular aquarium sponge. Find out if you have a glass or acrylic tank and use the appropriate sponge for that.
So that takes care of the glass. If you need to, use it to clean the overflow pipes or the sides of the aquarium, or clean some of the decorations.
You also want to control algae from growing in the first place. Algae thrives on sunlight and nutrients. And when I say "sunlight," I mean aquarium light, sunlight, any kind of lighting will promote algae. But if your nutrient levels are low -- your nitrates and phosphates, mainly -- algae's not going to grow that well. So the main thing to controlling algae, which will result in how you clean it, is to keep your nutrient levels in check.
But algae's going to grow inevitably. To clean it, you want to use the pad that I told you on the front of the glass. On the decorations, you can take them out, soak them in hot water. You can use light bleach, but you want to be very, very careful, because bleach is caustic. It'll discolor things if you use too much of it. And when you put the items back in the aquarium, if there's any bleach residue, it'll shock the system. It'll kill bacteria, and you'll have an ammonia and nitrite spike. But bleach is a great tool to cleaning plants. Only use it if you know what you are doing and are very, very careful. I don't want to hear people taking my advice, going and bleaching their aquarium decorations and parts, and losing their fish. That would make me feel really bad.
So you want to clean using hot water mainly. Use a good scrub brush. Not a wire brush, because a wire brush is going to just ruin and shred most aquarium decorations. And the glass itself will be kept clean just by doing periodic weekly cleanings with the pads.
Fish will also help to clean algae. Plecos, Otocinclus, algae eaters in freshwater aquariums. Also, snails will help to clean the glass and decorations in an aquarium. You don't want to use too many of them. Snails, if allowed to just multiply, will quickly completely overcrowd an aquarium to plague proportions. So just go with a couple of snails in the aquarium and keep them in check. And the fish, just a couple of small fish. Plecos, algae eaters. But remember that Plecos get really big, so if you have a small tank, maybe go with Otocinclus or algae eaters.
And if the water chemistry is kept in check, the phosphates and nitrates are kept low, the amount of algae that grows should be able to be kept in check with regular weekly front glass cleanings and the help of your algae-eating fish.