Model train track sections are connected by rail joiners that align the rails and carry the electric current. Both rails should fit in, not above, this joiner tightly. Joiners will be tight the first time you assemble the track. They will loosen with each reassembly. A gentile squeeze with needle nose pliers will fix a loose joiner.
If your track has attached roadbed, a second set of mechanical fasteners attached to the base helps secure the track. This is great for temporary layouts, but it does not replace the metal rail joiners. Make sure the rail joiners are aligned properly as you click the bases together.
If your track does not have attached roadbed, it is a good idea to secure it to the platform. Glue can be used but makes future changes difficult. Small nails are available at hobby shops. Take care not to hit the rails with the hammer or drive nails too far into the ties. A tack hammer and nail set work well.
Test joints by lightly running your finger over the top of the rail. If you feel any bumps, check the joint. If you missed the rail joiner, back the tracks apart and try again. Check the instructions to see how to unhook the base sections of your track if you have them.
Continue assembling track sections until you've completed your circle. There should be enough flexibility to fit in both ends of the last piece without any trouble.
Once complete, take a look around the track to make sure there are no kinks. You can also unpack one of the train cars and gently push it around the layout.