The Inspiron 3000 Series/3542 laptop is actually a wide range of laptops, with a wide range of features. We'll really need to know a lot more to give you relevant direction: CPU speed/type, RAM, HD especially. However, there are some no-brainer upgrades that will improve performance (and perceived speed) on any computer.
First step is RAM. The model has anywhere from 2GB to 8GB of DDR3L 1600MHz RAM, in a single stick. If you have 2GB or 4GB now, replace them with 8GB; 4GB is barely enough to get Windows running, and 2GB is just insulting. The limited RAM is made worse by the fact that the graphics module shares system RAM, rather than having its own dedicated memory. This means that when you're doing video-intensive stuff like games or movies, you have even less RAM to run the applications and operating system. iFixit sells a stick that will work, but the RAM is readily available almost anywhere.
Second step is storage. The Inspiron 3000 uses a 2.5" SATA III hard drive, but not a very fast one; the specifications say that all variations use a 5400 RPM platter drive. There are faster drives available, in order of increasing speed: 7200 RPM platter drives, hybrid drives (platter drives with very large RAM caches), and SSD (solid state drives). As you might imagine, the faster you go, the more the drives cost. SSDs usually have much less storage space than platter drives; a 500GB platter drive is well under $100, while a 500GB SSD can be over $300.
It's possible to get both the speed of an SSD and the storage space of a platter drive by replacing the original platter drive with a much-faster SSD, removing the optical drive from its internal bay and installing the original platter drive in that bay, using a special carrier. iFixit has such a carrier. You use the SSD as your system/applications drive, and store all your documents on the platter drive; that way, the things you're running all the time are on the fastest device.
Those are the easy options; after that, speed bumps get a lot harder. If your CPU is mounted in a socket on the logic board, you may be able to remove it and install a faster CPU. But most laptops don't use socketed CPUs; the processor is soldered directly onto the logic board. Even if you could replace it, it's not wise to do so. The cooling system in the laptop was designed to cool down the CPU Dell installed, not the faster/hotter CPU you put in.