Just gonna stick my 2p in here....
Keeping is not just something you can pick up over night.
just because you can catch doesn't mean you will make a good keeper.
i have had to field in a few t20 games and ive been asked to go to slip and gully etc and i cant cope with not being able to see the line of the ball.
iv'e done it from being 12 and my dad was a keeper to so guess it was in my blood. These are just my Views and what works for me. i've never had coaching just been left to get on with it.
It will take lots of hard work and learning to catch the ball different as you have to have much softer hands.
gloves do play a big part id always get really soft gloves.
like someone said on the 1st page cheap pads expensive gloves!
I am using b3's at the moment but they are on there way out

picked up some newbery SPS's but cant get used to them.
2 bits to keeping standing back and standing up.
Standing back is easier of the 2 you can see the ball all the way into the gloves and have more time to react.
Every pitch is different so where you stand to your bowlers will change.
A lot of new keepers i see not standing close enough and are taking the ball on the bounce or low to the floor. this makes the job harder.
Standing up requires more attention make sure you have a comfortable stance feet wide apart and rise with the ball .
your foot work is key here as is having soft hands.
i see some keepers when they are stood up they have there hands actually in between there legs when they crouch. This is not good as you need to move your hands quicky. You will find you have to pull them in front of your legs first then move them to catch the ball. your gloves should be in front of your feet and your body weight slightly forward.
To sharpen your reactions and to soften your hands practice with a tennis ball.
stand in front of a brick wall get a friend to throw the ball at it from behind you and use your feet and hands to catch the ball.
tennis balls will pop out often if you don't get it right.
leg side is pure guess work! you can only follow the ball so far once it goes down the batter will be in the way of the ball. so take a step down leg and try and guess the length.
leg side half volleys tend to be the hardest.
as for fitness i am 28 years old and weigh 24 stone.
I don't have any fitness issues with keeping 1st and batting second. but im guessing this is because my body is used to it.
I know when people have started keeping there legs ache for days after. you just have to get your body used to the added pressure on different muscles your not used to using as much