This is probably better off in the IG tactics forum... meh
Get the codex. I can't stress this enough. It should be your next purchase.
There is no real point telling you what doctrine you should get until you do, because you won't have a clue what were on about.
After obtaining a codex and reading through it thoroughly, I suggest reading through it again. Then select the units you like best and compose an army list and post that
As far as tips and tricks go, I would suggest that foot slogging it towards the enemy may not be the best of ideas as your troops will need to be stationary to fire any heavy weapons and there own guns to full effect. It may be wise to use the mounted units to travel the larger distances to the objectives (compared to the relatively short distances within your deployment zone) at a leisurely pace, as you don't want to get on top of the objective with little support. So timing the arrival to coincide with other units such as hellhounds, infiltrating and/or deep striking storm troopers, vet squads, and sws in order to hold at bay the impending assault of the enemies shock troops.
If you're intending to use the mounted units for the purposes of plugging gaps in the line it may be better to forgo the transports and use the points for more warm bodies to strengthen the line to begin with. Then again you could use the points for both troops and sentinels that way you have the advantage of portable heavy weapons with the luxury of more bodies. Though the choice is ultimately yours.
Hope these ramblings of a grizzled old soldier have given you some help. Welcome to the Guard