Jacqueline....I would call a professional but I am sure some of the folks on here can give you some great instructions!
Best thing is to rent a floor sander as well as the hand sander for corners. Rental place will sell you the necessary sanding discs or belts you will need. Finish the floor with your choice of stain and/or polyurethane. Might need two coats. It’s not hard but time consuming. The hardest part for me was getting the sander home. I had a VW at the time.
if you try to DIY, be very careful with the sander -- you can ruin the floor in a micro second by improper use .... going against the grain, leaving in one area too long, getting the disk/belt wet .... and the machines are very heavy and can create a LOT of dust -- even with a bag ...
should sand between each coat of Varathane (I used what they called gym spar held up a long time)
I rented a sander from Lowe's and then used 3 coats of Varathane on them. They still look good after a few years BUT wouldn't do it again. I found it hard not to get tiny air bubbles in the polyurethane as I was applying it.
If you want a very pristine look then go the sanding/varnishing route. I one redid a dining room floor by striping and finishing like you would a piece of furniture. It didn't look new- some marks and traffic patterns remained but it looked appropriate to a 50 year old house.
If it is varnish you will probably require a stronger stripper which will require a wash/rinse. Lacquer or shellac should be able to be done with a refinisher.
Once you make the decision to do something, you can do a small test ares.
PLEASE be sure to follow all safety and ventilation recommendations.
If you have a multi story home and can't carry the professional sander up stairs, you can do it yourself with a rotary sander and multiple sanding disks, starting with 40 grit, then moving up to 80, 100, 120, 220. You will require many disks, shop vac, good knee pads, anti vibration gloves, hearing, respiratory and eye protection, tack cloths, and great focus to work in 3' square sections, count your sanding strokes to insure you don't over sand any area, and many breaks to save your arms and back.
Tight areas can be done with "mouse sander" or by hand. If you have spaces between boards, sweep some saw dust into the spaces as a filler.
Tack cloth well before applying stain and sealant, taking your time to apply long, light coats (avoids bubbles), with a stain/sealant brush or long " Swiffer type" applicator.
You'll need good ventilation when applying stain and multiple coats of sealant.
Be sure to let coats dry completely between coats.
A room measuring 16'x 17' can be done yourself this way, over a few weeks.