Bike stores can simply do a normal "yearly" overhaul and put on brake pads, tubes and tires.
Road touring clubs and mountain bike clubs can be of great and personal help.
harriscyclery.com has fantastic documents covering almost every nature of cycling.
I have only ever found one error.
They state that mountain bike "slicks" tires work.
They do not.
Should you want to road with a mountain bike, substitute either Panaracer T-Serv or Kenda Kross Plus Yellow Label--because either are faster than slicks and can stop in the wet.
Do it yourself?
Sure, but don't bother with a bike that came from a "toy department". Just go to the bike store for some fun . . .quality. . . test drives instead, and bear in mind that they (the bike store) carry only fully functional bikes, so price has little consequence or merit.
On a good bike. . .
You need tires.
You need inner tubes, preferably "bike store grade"
You need brake pads, and I'd recommend Kool Stop.
The following chemicals:
Simple Green origional dark green formula
Singer Sewing Machine Oil
EPIC bicycle chain lubricant
Blue Magic car wheel polish
Formula 2001 car vinyl wax
Spray the entire bike very well with Simple Green; however do not get a concentrated blast of this into hubs, headset or bottom bracket.
See harriscyclery.com if you don't know where these are.
Now, spray it again.
Rub the bike with a washrag.
Rinse the bike gently with water. Do not get a concentrated stream of water into hubs, headset or bottom bracket.
Dry with a towel.
Set the bike upside down and leave it there an hour.
Lube the rear derailleur with EPIC.
Lube the front derailleur with EPIC.
Lube the underside of the bottom bracket IF the cables cross a bit of plastic there.
EPIC is non-harmful, so you can get quite liberal with it or even squeeze the bottle if need be.
Oil the chain with Singer Sewing machine oil, every link, rubbing it in with a rag.
With lots of shifting of both derailleurs and lots of pedaling, the oil will work its way into the chain and loosen up the rust. This will also put Singer Oil onto the cogs without the danger of getting it into the rear hub.
Oil the chain again with Singer Sewing machine oil and give it another workout.
Rub the chain with a clean rag and then lube the chain with EPIC. This will prevent dirt from sticking to the chain.
Discover all small moving parts and flood them with EPIC. Include cables!! When possible, get EPIC down inside the cable jackets.
EPIC will not harm hubs, bottom brackets and headsets, but those parts would not benefit from it.
Soak everything else that moves and then give them a workout.
Sometimes, a cable jacket may be easily tugged free from a socket that is welded to the bike frame. In this case, you can lube the entire cable with EPIC.
Rub the resulting mess clean with a towel.
Remove as much EPIC as possible with the towel.
Spray the bike quite terribly with Formula 2001 car Vinyl wax. Put it right-side-up and spray it again.
This will restore the emolliants that 5 years and the Simple Green have abused.
Wait 30 minutes.
Rub the bike with a towel in attempt to remove as much Formula 2001 as possible.
Clean the saddle with a wet rag and a dry towel. This will not remove the Formula 2001 entirely, but you just need to make the saddle non-slippery.
Put the bike upside down.
Release the brakes so that you can remove the wheels.
Remove the wheels.
Remove the tires and tubes.
Wax the wheels with lots and lots of rubbing and the automotive wheel wax, Blue Magic.
Now, wax the shiny metal part of the bike with Blue Magic.
Buff with a dry towel to remove residue.
Install the new tires.
Install the wheels back to the bike.
Install the new brake pads.
Go ride!
If the bottom bracket acts up (loose or click) or hubs seem catchy or crunchy or if the steering (headset) is squeaky or irregular, then these need to be greased. See harriscyclery.com for the instructions on those or go to the bike shop.
If shifting is irregular, look up the simple instructions for rear derailleur alignment or just give the adjustment collar a bit of turns to compensate for cable stretch.
Any remaining issues can be fixed by taking the bike in to the bike shop for its "yearly" and this will cost a lot less on a bike that has already had some care.
In five years, your body may have changed a bit, and if so, the bike shop can re-fit the bike for you.
Measure 1 foot of chain with the ruler. It should fit exactly 24 links into 1 foot of chain. If it is slightly over, buy a new chain.
If the chain is past 1/8th inch over, then the bike will require new gears and chain; however, since this has already happened, you may simply run it until it has trouble slipping. On the other hand, some people have reported success by immediately installing an SRAM brand chain onto a bike with slightly worn gears. I suppose that would work if it is not yet to the slipping stage.
For this reason, I'd recommend adding a speedometer to the bike, because they're fun and because they have an odometer. Chains last 1000 miles of gravel and 2000 miles of pavement before they become dangerous to the bike. The Schwinn branded speedometer at discount stores, such as Wal-Mart and Target are quite satisfactory as are the majority of that brand. You can also bet that the little battery will go dead slightly before the 2000 mile mark, and while the speed readout will become erratic, odometer will still have information saved. At that time, just replace battery and chain, and the bike will stay healthy.