A good rain cover will do a lot to keep your camera dry, but be aware that rain covers come in various sizes and designs to accommodate different camera models and lens combinations. Most are designed for use with pro DSLRs and mirrorless cameras with equally large lenses; smaller setups are likely to be swallowed by excess material, making them tricky to operate. So think carefully about buying a cover that's the optimal size for your camera.
Also consider functionality. Many rain covers feature transparent windows or flaps that allow you to access essential camera controls, such as buttons, dials, and the viewfinder.
As for long lenses, the average rain cover is best suited to a 70-200mm f/2.8 lens. However, some manufacturers offer longer or extendable versions for super-telephoto optics.
A rain cover won’t do much to stop droplets forming on the front of your lens. That’s a problem best solved by fitting your lens hood. Also, threading your camera’s strap outside a rain cover will get it wet and compromise water resistance. A cover with an integrated strap is a much better solution.