so lost!! please help!

hello there

i have the galaxy s3mini GT- 18190N
uk resident
carrier T-Mobile
android version 4.1.2
kernel version 3.0.31-1436784

ive had the phone for some time now and obviously internal space has became a problem so i looked around the net and it seemed that the first thing i must do was to root the phone....

i rooted with the main intention of being able to put my apps into my 32gb extsd card
secondly to perhaps upgrade the OS to say lollipop
thirdly to maybe try out some of these custom roms ive read so much about.

so i did root the phone using kingroot app, this seemed to help with the speed of the phone/reduced lagging and such but not with internal memory.

i then looked around for a solution and the general answer was to install link2sd and app2sd.

i installed both but had no joy due to no 2nd partition on sd card.

this led me to downloading rommanager (which apparently can create a 2nd partition on the etxSD) well this is when i realised my noobness knew no bounds, because for the life of me i cant seem to be able to do it lol

i looked around again and read as much as i could in regards to creating a 2nd partition on the extSD but again have not found a clear (complete and utter noob proof) guide to do this so am now stuck in limbo :(

ive read so much this last few days about flash/roms/internal sd/internal phone memory/external sd/nands/backups/backdowns/backflips ect. that i feel completely lost.

i understand this is probably a very tedious request to you more educated folk but what i would like to know is:

1 - can i return my phone to stock settings? (what it was like before i rooted) will this delete all the changes ive made and apps ive downloaded? and is this the best idea? to start all over again from scratch? if so how?

or can i download a custom rom that already has these settings? by settings i mean the ability to move my apps to the external SD? if so, how?

im really sorry for being a pain but my brains gonna explode.
please help

thank you