It's fine, don't worry, everyone has questions lol
With a profile pic you can go on your profile/settings and it gives you the option of putting a profile picture up (which goes on your page), and then an avatar (shows up on the forums). You can use default avatar's or upload your own but you need to crop it to the right size if you do.
Generally, pet store cages are far too small. C&C cages are better, bigger, and cheaper. You can make a lid for it out of grids by attaching them to the top all down one side with cable ties, this way you can lift it up and down easily. A lot of members do this if they have cats - see the gallery for some examples. But I would recommend building yourself a c&c
my boys are in a 2x4 with a 1x2 loft and they're over the moon - when I first got a guinea pig he was in a pet store cage and now I wouldnt ever put him in one again tbh, there's no going back lol
You can buy little nail cutters at the pet store or use normal nail trimmers, the ones you squeeze. Take off the tips and the tips only - else you'll cut the quick and your piggy will bleed. This can be stopped with a special powder you can buy, or corn flour. If your piggy has light coloured nails you should be able to see the quick, holding a torch underneath can help. If they're dark just do the tips. There are some helpful vids on youtube.
I tend to get my boyfriend to hold them while I cut their nails because they aren't really keen and it's hard to do when I hold them.
There's no set amount for bathing them, but you don't wanna do it too often. Noah's quite messy and managed to get dirty quickly so I generally bath him once a month, Elmo a lot lot less frequent. It depends on your pig/how long their coat is/etc. It's easier to spot clean them sometimes, I have to spot clean Noah's legs quite a lot. He gets smelly lol