Adding mulch to a garden helps stabilize soil temperature and adds nutrients. Applied properly, it also suppresses weeds, retains moisture and makes plants stand out. Knowing when to mulch can be a challenge. If it's almost too cold to spend any length of time outside, it's time.
Although there are a variety of mulches, I prefer oak leaves from my trees. My garden has very little grass so when it's time to mulch, I basically supervise where dry leaves drop. If I don't see an area adequately covered, I will bundle up to rake a pile of leaves to cover that spot but I pretty much let nature do the hard work. If in spring I can still see dried leaves, I use the remaining leaves as a guide to make sure the area is covered with seasoned, free wood chips picked up from our local recycling center. By the time we have our last frost in May, most of the leaves have been absorbed. Those still around get escorted to a composter.
I also like to mulch with straw, especially around my strawberries, lavender and my plant nursery where I grow garden samples. If you use river rock or gravel with a weed-suppressing cloth, make sure it's an area you don't plan to change for the foreseeable future. It's hard to remove that kind of mulch once it's spread. Perennial bulbs sometimes can't get through the cloth and rock basically deprives soil of light and nutrients so I would not use it unless it's a safety issue.
If you have an area with weeds, remove weeds by hand before applying mulch. Mulch will help suppress weeds and any new ones that sprout will be easier to pull. One of the best ways to remove weeds from a sidewalk or driveway is to pour plain hot water on them. I first pull the larger weeds, then add hot water to any remaining weeds.
Use weed-killing chemicals only as a last resort. Although they may seem to be a labor-saver, improper application can hurt surrounding plants and affect bees and other pollinators. Frankly I don't kill many weeds. Most are natives so whatever blooms and my bees like, stay. In spring, I also spread an organic fertilizer high in nitrogen to help balance the leaf acidity. And to think some people waste this free mulch by burning them!