There are several easy ways to do this but they can be dependant on how you designed your tubes to start with?
You can use each tubes std planes to mate them, e.g. front plane of one tube to right plane of another tube for example.
Another option could be to the tube axis's to mate each other if used the revolve feature for your tubes.
If you mean tubes joining as below your best bet would be to use Weldments (assembly below is not done using Weldments) which will do all the joints for you. You just need a profile sketch for the tube & a sketch for the framework/route. Take a look at the built in tutorials in SolidWorks or search online.
If it's not welded parts then you would have elbows & joiners that you would mate to using concentric, conincident & or distance mates.
Every cylindrical component has an automatic axis.
Turn on View Temp Axes to display them (they'll be blue)
Create a perpendicular mate for the two temp temp axes
As Rob Edwards mentioned above, you can use each part's plane to mate with:
(Remember to expand the tree and select within each part's tree to get their specific plane and not those of the assembly.)
The two tubes are perpendicular with their axes planar.
But with the horizontal tube fixed, the vertical one can still move, so you'll need to add more mates depending at the mating faces at the junction, as Kevin Pymm mentioned.
I hope this helps.