If you are in object mode (Tab, to switch between edit/object) and hold shift as you select everything you would like to join in to one object. Once everything is selected press Ctrl+j this will pop up options, click “join”.
Then you might also want to know how to separate these later on after you have the textures on, that way you can go back and edit the modeling. To do this go in to Edit mode, press a to select everything then press p this will pop up options select “Separate all loose”. And there you go all the non connected parts of that object will be their own object.
Also as a side note if you have any questions on how to do different things go to this post and look at my reply (#3). I put a list of different YouTube Users that have made and are making Free Blender Tutorials.
It will take some time and effort depending on the shape of the meshes and how you blend the different objects. You should be safe to join the objects into a single mesh temporarily.
The first thing you should do is create a vertex group unique to each object assigned to all the vertices in that object. This way when you join the objects into one mesh you can still select the components and separate them again (“p” key).
Depending on how you do the unwrapping, you might need to remove double or merge vertices. In this case you might have to rip the edges again (“v” key) and adjust your vertex groups.
There might be a simpler way to do this, but it seems to me like it will take some effort.
Before you Join the meshes, go to each mesh, select all it’s vertices and make a vertex group.
Next join all the meshes and the resulting mesh will retain the vertex groups of each part! Wish there was an automatic way to do this like join and create vertex groups.