This is quite simple with screenshots and Photoshop. If you are bootstrapping, try resources like fivrr.com, elance.com, or odesk.com

Otherwise, ask your friends/family/neighbors if they have a copy of Photoshop for you to do this.

If you'd like, setup a call and I can have my in-house designer do it for you.

You need to provide more information about what tools you are currently using.

For the purpose of creating wireframes, I would suggest using Balsamiq, and avoid doing any styling at all. This is good for the phase when you are working out screen flow and layout.

If you are doing a mockup, i.e. something that can look like a screenshot and will determine the graphic style of the app, then you really need to work with a graphic designer. Would you ask a non-programmer to write code? It's the same with graphics -- there's no substitute for a professional. At the stage you're at, you should be able to find an independent freelancing designer quite inexpensively in any decently sized city.

If you mean the first time you run an app (or enter a website) the whole screen grays out and you get writing and arrows explaining it: don't. Users /hate/ it.

I have a little experience in this, but have talked to others who did a lot of research into it. In this case we're not just talking about reducing satisfaction, but actually increased abandonment. 4-8x more people don't finish the tour, then delete the app (similar issues for websites).

You are better off, always, just making the functionality of the site more apparent off the bat. You CAN demo stuff, but it has to be inline with their actual use. Help can work to allow users who want more info to pull it to them, but is fraught with peril also. Be careful not to imply they need help, because they aren't smart enough, etc.

That said, I have had to make them a couple times, so may be able to point you the right direction at least. Is the question one of design or coding?