If i spend some time leveling smithing to make my self better armour etc i understand that i also gain levels, how much of a difference does it make to the game? say i level smithing up about half and get to 10th and then start the game proper will it make much difference to the game.

some, but not very much. I would find some time to get some offensive and defensive skill in the mix though before just ramping smithing.

put perks in your one hand or two, also in light armor or shield if you use them. The faster you level smithing and get perks in there the better in the long run since you can make and improve your armor, improving magical (enchanted) armor is a must have. Light armor is the left side, heavy is the right side of the constellation. Put perks in enchanting later, but you can level it in the meantime. I also put perks in archery.

You want to try to keep your combat abilities in line with your highest skills. If you just spent your first levels buffing everything but combat skills, you will get your ass handed to you by the now levelled enemies.

However, levelling Blacksmithing first can benefit you. You won't have the damage through skills but you will with superior armour and weapons.

This is good advice, especially if you're leveling enchanting or alchemy with smithing or vice versa. I have 60+ enchanting and 50ish smithing but low offense and defensive skills/perks. Game is hard kinda hard right now, be careful how you go about it...

One thing to keep in mind is that you won't be able to gain a level by raising the same skill over and over again. I'm not sure exactly how it works, but after a few skill increases, it will stop contributing to your overall XP, so if you're only concentrating on Smithing, you may not be able to level up as much as you think.

Even so, I don't think it would make much difference. I didn't really start doing the main quest until level 25 or so, and I haven't noticed any problems with difficulty that aren't already present in the game anyway.

Being a level 10 or 20 Blacksmith on Master = no fun. Level yourself gradually, you'll have a much nicer experience.