New to photography, could use the help!

So I’m just starting out in taking pictures and I know how some things work but could use some pointers. I purchased a fairly cheap Cannon XSI , with a basic lens on it (maybe 35-75mm?)

I want to get a 100-300mm lens , will I be able to take some pretty good pictures with that? I’m not trying to shoot for Vogue or anything just some portraits and landscape stuff but I know the body is pretty old (2008) so I wasn’t sure because of the processing power.

Also, are all neck straps universal pretty much? Because I wanted to order this really nice one I saw and I don’t know if all straps pretty much can fit all cameras. Any tips would help thank you!