Hey guys,
I'm a panel beater by trade and have done a few motorcycle tanks before, it's a fairly easy but fiddly job. Metal bars and a large mallet/hammer come in handy but not for where the dents are in that tank, it's too hard to get any purposeful blow onto the inside of those dents through the filler hole at that angle also it is not the preferred method as most modern day tanks have baffles of some sort. Cutting access holes in the tank and welding them back up is also out if you can help it, that's just rough. Using just big and not getting the dents out first, you will have some pretty big high spots around the repair.
There's a few things you will need to repair that tank-
Garden hose
Rag
Paint stripper
Sander
Welder (spot or Mig)
Box of washers
Small slider hammer
Grinder
Bog
Sand paper

1. Shove the garden hose in the exhaust pipe of a car and shove a rag around it to make a nice seal and put the other end in the fuel tank, leave the car running for 2 hours. This will void the tank of all flammable gasses as we don't want to make an explosion later on.
2. Using paint stripper and a sander, clean the area up real nice to a bare metal finish.
3. Using the welder, just with a very small tack, weld a washer standing up on its side into the dent.
4. Using the slide hammer with a small hook attachment through the washer, just lightly tap the dent out. It's easiest if you start towards the outer edge of the dent and work your way towards the centre. Just be gentle because you don't want to go tearing holes in the tank that you will have to weld up later.
5. If your tacks are small enough, you should be able to to just rock them back and forth till they snap off, if not you will have to grind them off. I use to do my welds small enough that one good hit with the slide hammer would pull the dent and break the washer off at the same time. You want to get as much of the dent out as possible without creating any new high spots, you may also need to gently tap around the dent with a mallet while putting some steady pressure on the washer with the slide hammer, this will help to pull out the dent and get rid of the high spot all around the dent.
6. Clean the area up again with the sander and possibly the grinder if needed, always finish with the sander, the bog will hold better.
6. Bog away.

It requires a little time and patience but you will be much happier with the finish.