I want some basic things in toolkit (like oiling of chain, tightening of brakes etc). Where can I find them and how much does it usually cost? I want to buy one, so can you name the basic tools with their functions in layman's language?
You can easily find a "bicycle multitool"/"multi bicycle tool"/"bike multifunction tool" that covers all the basic essentials in one tool. There's tons of brands and models and what's most readily available may vary regionally, but common brands I've seen are: Park, Crank Brothers, Topeak, Serfas.
Where? I would go to my local bicycle shop to get them, or there's plenty of places you can order them online. I have no idea what Walmart stocks. REI, Amazon.com, jensonusa.com, etc. The only place recommendation I'd go with is: your local bike shop, that's owned by people into bicycles and primarily sells bicycles; they're bound to have some tools for sale.
Except for the pump and rags, a typical bicycle multitool will have all those and cost between 20USD and 40USD. A "standalone" version of each tool will almost always be better than what's included in a multitool, but more expensive to get a whole set. However, you can get a decent set of hex keys for under 10USD and tire levers for 5USD and still be a bit cheaper than a multitool.
Really, I think that covers the basics. You might also consider:
Less important / Less "basic"
The bike toolkits you'll find at Walmart and the like are near worthless -- the tools are poor quality and frequently not really usable. (Eg, the "dumbell" wrench that's often included is too bulky to fit in many places where you might need it.) Either get a multitool made by one of the mfgrs mentioned in Freiheit's post or collect the individual tools you need, bought at a good hardware store, with the few specialty pieces bought at a bike shop. Keep in mind that you don't need tools that don't fit your bike (but you may need, eg, two copies of one particular size of wrench in order to tighten some fastener on your bike, so even with the multitool you may need a few other pieces). And don't scrimp on tools -- a poor-quality wrench can wreck a bolt to where it can't be removed from the bike.
In terms of oil, don't use ordinary motor oil or "3 in 1" -- buy purpose-made bicycle chain oil at a bike shop. The bike shop people can describe to you the differences between the various types. For cleaning your chain consider getting a chain scrubber (along with the appropriate chain cleaner fluid).
If you want to carry tools on your bike, the best way is with a tool bag that straps under your seat. You can actually improvise this with just a maybe 16" square of canvas and some buckle/Velcro straps (just roll up the tools into a bundle), though a purpose-made bag is nice. For the tools that stay home, a simple plastic box or a "tool roll" is often a better choice than a compartmented toolbox.
For removing and installing tires I'm partial to the Quick Stick rather than conventional tire levers -- much less chance of damaging the tube, and generally a lot faster. However, it won't work on some particularly tight tires.
In terms of how to do things, Park Tool Company has an excellent web site that covers virtually everything you might want to do.