It's best to first know why you want to learn web development in the first place. If you're looking to build websites for local shops in your area, then only knowing front end may be enough. If you want to build your own web application and launch your own platform, then back end may be necessary first to begin your project build. Personally, I find front end better to learn first as it's easier to see the results of your work. This helps you build your confidence in programming and better understand how to make things work on the back end when you know how they need to look on the front end.