The best harness (is this what you mean by strap-on?) for the beginner is very similar to the best harness for the advanced user. You want one that is sturdy and fits well.
Consider one from Sportsheets, Aslan Leather (the Commando is affordable), Outlaw Leather, or SpareParts.
Then match it with a silicone dildo with at least 5" of insertable length.