The setting that controls whether <Tab> and autoindent insert tabs or spaces is
" spaces
set expandtabs
" tabs
set noexpandtabs
This works in conjunction with tabstop, softtabstop and shiftwidth settings (you most likely want to keep these equal):
set tabstop=4 softtabstop=4 shiftwidth=4
You can also abbreviate all of these:
set et ts=4 sts=4 sw=4
Once your options are set, you can convert the current file's unwanted tabs to spaces (or unwanted spaces to tabs, if noexpandtabs) by issuing the :retab command.
Using a modeline in each file (:help modeline) or a project editorconfig with the editorconfig editor plugin are advisable to avoid this situation in the future when sharing code with individuals with unknown editor defaults.