Overleaf is a cloud-based LaTeX editor and is free to use. The free version allows you to create, edit and share your projects without having to install anything on your computer. Though there are options to upgrade and pay for Overleaf, the free version includes:
It also includes over 4000 packages, but you can also upload your own .sty or .cls files for other packages not included.
With Overleaf you can login to your account from anywhere, and the free version allows you to privately collaborate with one other person for each project.
To get started, head over to Overleaf and register for a free account. From here you can either register with your email account or click on the "Register using Google" option if you want to link it directly to your account.
Once you have registered you will be prompted to start your first project, You can choose from one of the options in the dropdown menu, or just choose to create a blank project. There are also a variety of templates available including bibliographies, books, resumes, posters and much more.
Once you create your project, you can add a new folder, new file, or upload files with the options available in the left corner of the screen.