Hello and welcome to the forum. Generally most struts are sold as just the strut - not premade assemblies with springs, though they can be purchased in that way.
You will need a spring compressor to compress the spring - I would highly recommend the clamshell or heavy duty variants (many can be rented for a small fee) - the cheaper claw/hook variants can be downright dangerous to use. Ideally - you want to replace struts in axle pairs, otherwise handling could be adversely affected.
If you do a search for springs - there are several discussion where we discuss some tips and things to watch out for while doing this job. Well within the scope of a DIYer with the right equipment, but some items - like the rear brake line, or rear swaybar endlink - can be tough to get around.