Heres a few tips for when closing topics.
Close any topic when its pointless to continue it. For example, if a topic asks for help and the person ended up getting the help, than you can close it when help has been satisfied from the person or if the person got the help, but if people are still posting in it, than you can close it.
Some topics like the one I created thats called 'Treasure Map Locations' or this one, may want to be kept open because its a very open topic and can be used for other people to state any problems or statements on the particular topic.
If a topic has people who are has Less than three words or is not posting anything on the same topic (Spamming)/ breaking rules etc. Than delete their post.
If you can do all this, than you'll be a good mod.