I assume you live in the USA? If so, then yes you can make a silencer legally without a license as long as you live in AL, AR, AK, AZ, CO, CT, FL, GA, ID, IN, KY, LA, ME, MD, MS, MT, NE, ND, NV, NH, NM, NC, OH, OK, OR, PA, SC, SD, TN, TX, UT, VA, WA, WV, WI, or WY. A few other states allow them if you have a license (FFL/SOT) to deal or manufacture them as a business. You must be at least 18 to make a silencer or buy one from a person in your state; 21 to buy one from a licensed dealer.
The procedure is simple, but time consuming. Print the ATF form 1 and form 5330.20 from the ATF website, or order them from the ATF. Fill out the ATF form 1 and bring it to your local sheriff for their signature. Send it in to the ATF along with form 5330.20, your fingerprints, photos and a check for $200. It takes about 4-6 months now for approval, but the ATF approves every form that is filled out properly. Your approved copy of the ATF form is not a license or permit, it is just proof that you paid the tax and legally possess the silencer. It is the only proof that you legally own the silencer, so it must not be lost.
Check out the forums at http://www.silencertalk.com and http://www.subguns.com for advice. While it is simple to make a good silencer that reduces noise by 20-30 decibels (100-1000 times lower), you will need a lathe to do a good job. I learned to use a metal lathe while making my first silencer. Here is a drawing of a silencer I made. http://i171.photobucket.com/albums/u320/ranb40/suppressors/22lrsuppressor-1.jpg It is just a straight metal tube with K shaped baffles in it. Two end caps hold the baffles in and attach it to the barrel with threads. Very simple.
Soda bottles, potatoes, condoms, pillows and other dubious objects work poorly as silencers. Anything that touches the bullet while it passes through the silencer can lower speed and ruin accuracy. Use of the above improvised silencers can damage the gun and injure the shooters or others nearby. Anyone suggesting that you use these things is stupid.
Do not expect a silencer to make any firearm silent. While a good silencer can reduce noise by 30 decibels, even a 22 caliber rifle will still be about 115 decibels when suppressed. This short duration impulse noise will not sound loud at all though. Firearms shooting subsonic ammo (<1100 fps) will be the least noisy. High powered rifles shooting supersonic ammo are louder, but can still be hearing safe when used with a silencer.
If you want to buy one instead, they cost $100 to $400 plus the $200 tax. A dealer can guide you through the ATF form 4 process for buying one.
Ranb