Those are pretty open ended questions, and the best I can do is say it depends.
What kind of gun are you trying to suppress? Do you need to suppress multiple calibers with one can? Do you have any size requirements (must fit underneath a AR handguard, must be as short as possible)? Do you need it rated for full auto? Do you want it to be as quiet as possible or do you just want something that's hearing safe? How much do you want to spend? There are lots of variables and what may be the the perfect can for one person in one situation, may not be right for the next person.
Process wise, you'll need to find a SOT (special occupational tax payer, basically a FFL who pays a yearly tax to also sell or manufacturer NFA items) who either stocks the suppressor you want or who can order or transfer one to you. If he has it in stock, you fill out a Form 4 for the transfer, send it off with a $200 check to the ATF and then wait. Eventually the form will come back approved and you'll be able to take your suppressor home.
For a first can, I would look at either a full sized 9mm can, or a .22 can. That will give you the best suppression and will be a good general use can that will serve you well. In my opinion a good stable of cans would be a full size 9mm can, a .30 caliber can, and a .22 caliber can. After that you have most of your bases covered and can get into more specialty cans. Then again, it all depends on what you want to do. I personally started off with an "exotic" 9mm micro can. If I were to buy another full sized 9mm can, it would be a Liberty Mystic, a very quiet can that is rated to work an a bunch of different calibers.
I have two cans and am waiting on a third. My first can was a Thompson Machine Poseidon, a 9mm micro can. Where most full sized 9mm cans are around 8" long and use baffles, this one is about 4" and uses a mix of a monocore and a wipe to seal the can. One downside to this can is that the wipe needs to be replaced every 20 shots or so to maintain maximum suppression. It is a fun, cool, and unique can, but not the most practical for range sessions.
The second can is a .30 caliber by HTG. Unfortunately I haven't gotten a chance to shoot it because its host is a SBR in .300 Blk and I can't find any ammo for it
The can that's still in the transfer process is a HTG 9mm full sized can. Haven't gotten a chance to shoot it yet, but I'm looking forward to not having to swap wipes.
I managed to get very good deals on these suppressors because I bought them from dealers who were getting rid of cans because they either got them as demo suppressors, or because the manufacturer was going out of business and they were having a clearance sale on their stock. If you have a good idea what you want and know where to look, you can find some very good deals on suppressors.
If you want more than one get them all at once that way you only have to wait one time and it makes it a little less painful!
Or you can buy them one or two months apart, and have a constant stream of NFA toys showing up at even intervals
