Virtually every user-serviceable problem can be solved by one of the following:

Reading the fine manual and making sure the machine is threaded correctly.

Replacing the needle.

Adjusting the tension, starting with getting the bobbin tension grossly correct, then balancing it with the top tension. <- This is not hard; it's just that most people haven't been taught[0].

Removing the accumulated lint from the bobbin driver and feed dogs.

Lubricating the machine.

If none of those work, have it serviced. If the service person tells you the machine is crap, go to a thrift shop and buy a Singer 66, 99, 15 or equivalent Japanese clone for $25-$100. For a little more money, you can get a 201. A Featherweight is a joy to use and takes up no space in storage, but is much costlier than any of the above options.

Don't buy a slant shank machine (400 or 500 series); that was an evolutionary dead-end. If you absolutely need a machine that zigzags, ask the service person what they recommend.

[0] This is applicable to Singer class 15 machines and their clones, but the general principles apply to any lockstitch machine:

https://ismacs.net/singer_sewing_machine_company/manuals/ha-...

If you have a transverse shuttle (you almost certainly don't) or a vibrating shuttle (you probably don't), you may need to look up information specific to your machine.