Hold your non-dominant hand out in a flat handshape with the
fingers together except for the middle and index finger. Place the
dominant index finger at the base of the non-dominant index and
middle fingers and twist the dominant index hand.

BEGIN or START:



Notes:
Also see:
BEGIN
Tip: In ASL the sign WITH is not needed when using the sign START in a context
such as: "...start with the letter ..."
In most situations, the sign START means "start with" / "start to" / "start
from" and doesn't need an added preposition.
For example, if you want to sign "Five words that start with the letter M" you
could sign: "FIVE WORD START LETTER M..."
If you are in a low-context situation you can expand this sentence by adding
signs such as "EACH" or you could add at the end a type of example by signing
"M"+"FINGERSPELL," lowering your hand a bit and again signing "M"+"FINGERSPELL,"
-- thus creating a type of "sample" list.
This is similar to not needing the sign "TO" when signing "GO."
In ASL, "GO" = "go to."
If you mean "start" as in "to start a car or vehicle with a key" you can depict
that by miming the turning of a key in the typical location to the right of
where a steering wheel would be in relation to your body.
turn-KEY / start vehicle / lock or unlock a door:

Memory aid: The sign for "turn key to start a car" actually makes a
pretty good memory aid for the sign START since both signs twist.
Notes: