I have just started a campaign with only one player. So far its going well but I fear combat may be a problem later on. Any tips toward what I should do?

One option would be to make them more of an Epic hero, give them X number of actions per round, instead of just one, things like that. The action economy is what you really need to be concerned with, IMO, since in a usual party they can take on anything one v one because they have an action economy advantage. Take that away, and a one v one fight is going to become a bloodbath at higher levels.

Ok thanks good idea.

I do two things.  I calculate the PC's CR as though they were a monster, erring on the conservative side where reasonable (Second Wind adds to effective HP, but I don't assume Action Surge will be used and I don't assume Heavy Armor Master or Superior Critical will apply), and use that to determine how much to throw at them; and I make them stronger than I would if there was a full party. 

I used to give them a bunch of "DMPCs," but I'm pretty sure that's why that game ended; a party mostly full of NPCs tends to lead to player boredom, I believe.