Black powder basics

 Yes, make sure there is no charge currently in it.

  I would use 777 powder 1st off, Pyrodex as the next option.

It should also be mentioned, you need to know with 100% certainty that there is no charge currently in the barrel.  This is best accomplished by a gunsmith.  

Looks like your Hawkins likely has a 1:48 barrel twist.  If it's different let us know.  That twist rate is a compromise between faster twist for sabots/conicals and the slower twist for round balls.  With enough tinkering you would be able to develop loads for both that shoot good, but being new to this, I would save that for later. 

Since you said you will be shooting 20 yards, I would concentrate on developing a round ball load.  Main reasoning being there are so many less variables in bullet choice and they are likely easier to find.  A round ball load in your twist will require a lighter powder charge, but at less than 50 yards, and even further, will be plenty. I would start at 50 grains and work up from there.  

Other than that, the other advice you got is good. Only thing I would add is to keep your barrel clean while shooting and develop a pattern at the range. Black powder, and even the substitutes are dirty stuff.  I used ffg and swab my barrel after every other shot.  A fouled barrel makes loading difficult and accuracy poor.  It's a fun hobby, and if you know someone that can help you at a range it will increase your learning curve greatly. 

I have had good luck with loose 777 I prefer the FFF powder over the FF. Another clean choice is Blackhorn 209.   In a sidelock you are probably only going to shoot 90 gr max of the 777. You may find 30 gr pellets that will work for you or get 50 and 30. I really have never had an issue measuring powder and find I can dial in better.

 On my sidelocks (hawkins) I still use the hot water cleaning methode and coat the barrell with bore butter.

I would guess the primer is a no. 11 or a musket cap.

Of course you need .54 cal bullets. I have found powerbelts to shoot great but not hold together on deer. In the .54 you are getting more limited choices as they have become less popular than the .50 cal

If you can find a friend to show you the ins and outs. Most important always make sure the bullet is seated.

If you are in the Mukwonago area I would be happy to help you out.