Firstly you can download a manual from the Singer website, link is
, type in "W750" and click search.
Once youve got the manual downloaded, I'd suggest you print it out and get it bound in covers so it can stay with the machine.
Then trouble shoot the tension, this sounds to me like you've got insufficient or no tension on either top, bottom or both threads. You need to revise the threading up and make sure you always thread up the top with the pressure foot raised so the thread properly seats into the tension discs. Set top tension on the normal mark between 3 and 4 and make sure when you load the bobbin into the bobbin holder, that the thread gets clicked into the tension spring completely.
Now test sew again, and tighten up the top tension a fraction if it shows on the underside. You can't adjust the bobbin tension on these machines or at least the manual doesn't show any adjustment, just says to adjust the top thread tension.
As a rule of thumb, change your sewing machine's needle for each new sewing project too as this will help with stitching quality.
If you can't get it looking any better consider taking the machine back to the shop (if its fairly new) and getting it checked.
If you are planning to sew lots of things, you may want to consider buying a higher level quality machine than this one, it is an entry level machine and there is huge variation in stitching quality between the cheaper models and the higher end quality ones. Just depends on your sewing needs and desires I guess as to whether its worth investing more $$.