Keeping my instrument in tune is one of the most difficult things I've learned. And I didn't start getting good at it until I owned a tuner. It is essential to use a tuner when you are learning a stringed instrument.
Strings are a complicated thing to advise on.
Some like 'em thick and weighty, others like noodles. Strings also lose tone. I'm not sure that I understand it completely. For sure, the rust on the string inhibits vibration and effects the tone. But there is something else as well.