At a minimum lube the chain and see what is scraping.
For the brakes just hold the bike up with one hand and spin the tire.
Visually inspect if the rim is rubbing on the brake.
Some times you can just adjust the brake and some times the wheel(s) need to be trued.
Some times you have the tire rubbing on the brake.
At this point need to decide if it is worth fixing.
Can the wheels even be trued?
A decent set of wheels is $200.
A good complete overhaul at a bike shop is over $100 and more like $200
If you need new parts can easily get over $400
Get an idea of how much it will cost before you start spending money
If you don't want to spend money then
- take off the tires and chain
- put the wheels (without tires) back on
- leave the brake released
- do the wheels spin freely
- does the crank (pedals) spin freely
- does the headset turn freely
At this point if things look okay and you don't want to spend money then repack all (unsealed) bearings. If you don't own any tools then probably not worth it.
If you need some new parts you will find out. Add em up and see if it is worth it.
A cheap bike in need of a lot of maintenance is typically not a good value.
You are typically better off spending more money on a bike that does not need much maintenance.