Got your manual? Let's try this from the beginning.
First, take all the thread off of/out of your machine. If your thread is the three spools for $1 sort, go get a spool of "dressmaker thread" of a name brand -- Coats & Clarks, American Efird, Gutermann, Mettler, Superior, etc. Crummy thread can cause a lot of sewing problems. See the photos here: http://sewing.about.com/library/weekly/aa102100a.htm
If you're using crummy thread right now, strip the thread off the bobbin, too, and wind a bobbin of matching thread.
Take a look at the spool of thread... if there's a little nick in one end meant for holding the end of the thread in storage (Coats, American Efird, other level-wound brands) put the nick where it won't possibly catch thread as it pulls off the spool -- usually on the top on a vertical spool pin, to the right on a horizontal spool pin. If you're using a vertical pin, there should be a little felt "doily" under the spool. Make sure there are no gummed labels on the thread that have torn and are sticking to the machine.
Let's also put the machine back to "default" settings, so put in a new size 80/12 sharp or universal needle, and the thread tension set to "4". It is not unknown to have a defective needle breaking the thread, so let's stack the deck in our favor with a new one. Make sure the needle is correctly oriented -- check your manual for which way the flat back of the needle should face. If the bobbin is towards the front of the machine, the flat goes to the back. Make sure the needle is completely pushed up into the needle clamp.
Clean out the area around the feed dogs and under the bobbin case of any stray threads, lint, needleshards, bits of pin.
Take a piece of plain cotton cloth (hankie, fold of sheet or such) moistened with rubbing alcohol or unflavored vodka and "floss" it between the tension disks to get out any bits of lint or coating that may have built up.
Wind a new bobbin, if need be. If you're using polyester thread, don't wind pedal to the metal... it causes the thread to stretch and then the seam puckers. Wind the bobbin at medium speed, making sure the thread is winding flat, not loopy. Put the new bobbin in the bobbin case, following the manual.
Rethread the top of the machine from scratch, following the manual, making sure the thread takeup lever (the thing that bobs up and down) is fully in the uppermost position, and that the presser foot is raised so the thread can enter between the tension disks properly.
Use the handwheel to raise the bobbin thread. Pull out about 3" of both top and bobbin thread and pull them under the presser foot and behind it.
Insert your fabric under the needle. Using the handwheel and turning it in the correct direction for your machine, drop the needle into the work, and then drop the presser foot. Hold the threads together behind the presser foot and make 1-3 stitches using the handwheel. Drop the thread and you can start sewing normally.
What's probably going on is a slight misthreading -- there's a loop of thread backlashed around a spool pin, the thread's caught in the keeper nick on the spool, there's an extra loop around one of the eyes, the takeup wasn't properly positioned... lots of possibilities, all of them hard to spot. Which is why when I have a machine that balks three times on me, I don't even try to figure out what's going on... all the thread comes out/off the machine, I clean, oil, put in a new needle and rethread. 99% of the time, that fixes it.
The other possibilities are that you've got a snag or sharp spot somewhere along the thread path. Generally that can be spotted by flossing with some silk thread or a piece of pantyhose nylon, and then the snaggy spot polished out. That's pretty uncommon, though, unless you've been doing something like running Nylon #69 upholstery thread in a home machine.
The other possibility is that you've got a thread jam forming at the beginning of the seam because of incorrect starting methods. The "hold the tails together" method of starting fixes that, and it quickly becomes a habit.