As a general resource, Park Tool has a great collection of service and repair guides. They list the tools you need for each type of repair.

Here's the basic maintenance checklist for an old neglected bike:

• Inspect anything rubber, and replace as needed. Tires and brake pads harden over time.
• Drip some thin lube (Tri-Flow or light oil) into the cable housings. Run the cables through the housings to spread the lube throughout.
• Lubricate all the pivot points on the derailleur(s), as well as the axles of the rear derailleur pulleys.
• Lubricate the pivot points on the brakes. Be careful not to get lube on the brake pads.
• Lube the chain and check chain "stretch" using a chain gauge. Or just replace it and start fresh.

And if you really want to get into it, here are some more advanced maintenance items:

• Repack the wheel hubs with fresh grease.
• True the wheels.

To build a basic toolkit, start with this:

• Tire levers
• Phillips screwdriver, size 0 or 1.
• A small flat-head screwdriver.
• Metric hex/allen wrenches. 4mm and 5mm sizes are the most common. Get a set that goes from 1.5mm to 10mm. Having ball ends on the 4mm and 5mm wrenches can be a lifesaver.
• Metric box-end wrenches. 8mm, 10mm, 13mm, 15mm, 17mm are all common sizes.

More advanced maintenance will require special tools specific to the parts installed on your bike. The designs of freehubs/cassettes/cranks/bottom brackets have all changed a lot over the last few decades.

Youtube channel of the company park tool has some great videos for repairing almost anything on bikes.

Link: https://www.youtube.com/channel/UCzaZ1sPWEuZN-I8_XT6AH8g

Used their videos when I started to work at a bicycle shop to teach myself how to repair some things and I still do rewatch some of the videos occasionally when I need to refresh my mind.

Just want to add some things. You should also visit https://www.sheldonbrown.com for more detailed informations. Good tools are helpful, its better to spend more money for quality. A repair stand, even diy, makes things easier.

Is it an older bike (vintage) or some that was purchased in the last 5-10 years?

If you don't have any tools, I'd personally suggest getting a bike tool set, which should cover much of what you might need. I would normally just buy whatever tools I need for the task as hand, and build up slowly, but it can be annoying if you don't have something.

Tools are an investment, so if you don't plan on doing bike repair after this one is done, it may be cheaper to send to a local bike shop to get worked on.

The youtube channels RJ the bike guy (vintage bikes) and Park Tool are some of the best I've used. But I'll often search for info and see what several channels say, since some techniques can differ slightly.

Well the first question is, if you throw lube on the chain (and everything that touches the chain like all the gears) and put air in the tires and then take it for a spin, what feels wrong? The brakes? The shifters? Do the tires hold air? Are the brakes dragging even when you don't touch them? Can you shift through all the gears without unseating the chain? Those are in my experience the common afflictions of a neglected bike, but if its been kept dry it might be surprisingly rideable.

@SirNuke See if there’s a local bike project near you. There’s one near me that accepts donated bikes and refurbishes and sells them for fairly cheap or uses them for parts. They have all kinds of tools and workspace that you can use to work on your own stuff.

If you're more into YouTube video tutorials, check out RJ the bike guy

Good suggestions here. I also found "rj the bike guy" on YouTube to be pretty good. He has a few videos of getting trash bikes back to ridable state

Park Tools is an excellent resource, as others have mentioned. In addition to the videos, I can highly recommend the Park Tools Big Blue Book of Bicycle Repairs.