There are lots of different options available for joining your blanket.  All of which will affect how your blanket will look and feel.  Which to choose depends on the look you want to achieve.  Here are a few of the popular ones to help you get started.

Whip Stitch
Sew your motifs together using Whip Stitch

Mattress Stitch
Use this invisible stitch to sew your motifs together

Crochet Joins (From the Front)
Crochet your motifs together to create ridges

Crochet Joins (From the Back)
Crochet your motifs together to mimic sewn seams

Join As You Go - Rounds
Join the squares as you make them to save extra work at the end of your project

Join As You Go - Rows
Joining crochet together as you work back and forth in rows.

Join Variations

Join As You Go - Daisy Puff Hexagons
This uses exactly the same technique as described in the the Join As Go tutorial above, but with little Daisy Puffagons

Lacy Sampler Blanket Join
I used this variation on the flat braid join and join as you go techniques to attach my Sampler Blanket Squares