Any bike recommandation for a beginner?

Anthony, welcome to this side of the fence!

The best I will recommend is some of the newer bikes from Tamiya. (For Harleys I can't help you off the top of my head, but I've seen some kits around.)  Everything from the RC211V's (Honda MotoGP bikes) onwards are just great. Goes together much better than some of the earlier bikes, fits are snug - I'd say oerfect for a first attempt.
Not aware of your modelling skills, esp on the painting/masking front. If that mat be an issue, keep it in mind for certain of the liveries! You don't need any special tools except maybe for the correct TS paints, because some of the MotoGP bikes have very specific colour requirements - if you go that route, that is. They also have some awesome roadbikes recently released that you can, of course, paint your favourite colour.

Personally, I (and a lot of the others) mostly remove the chrome plating completely and respray, which takes that issues out of the way and gives you a closer to real finish. However, you can touch it up with something like Tamiya chrome silver (x32?) paint if you don't want to go that way...

If you want to, you can also have a peek over at GB's - there is both a MotoGP GB and a street bike GB running.

Good luck and enjoy! Ask if there's any questions!

Ditto to what seabee said. 

As far as Harley's, I've seen Testors' ones in my LHS and on ebay.  If my memory serves me correctly I think they were 1/9 instead of 1/12.  They have the finished model on the boxtop and honestly they weren't that good with softdetails, horrible seam lines and all.  The best kits you can find are probably those 1/6 Tamiyas thought you only get the older FLH and a superglide and of course if you can pay for the extra premium there is a Fatboy released just to the japanese market. 

Something close to the big V-twin/chromed everything harleys will be Tamiya's 1/12 yamaha roadstar and Aoshima's 1/12 V-max.  Both are decent kits. 

One minor warning if you are thinking about the new Tamiya GP bikes, even though they are darn good if not incredible, they will need you to go for a paint shopping spree from the Tamiya paint lines for colors like titanium silver, titanium gold since 80% of those paint colors weren't needed on an aircraft kit.

Welcome to the two-wheeled world!  In addition to what Archangel said about paint, sometimes your LHS may not carry Tamiya botle paints, a problem which I've been having lately.  I've found that Testors Model Masters are really good substitutes for a lot of the metallic or composite colors.  I've heard good things about the metalizers also, though I haven't tried them yet.  I'm willing to bet once you build one or two of the Tamiya bikes, you'll think twice about going back to aircraft.  That's how I started, anyway. 

don't forget your helmet   

Thanks guys. Thank you so much for the kind suggestions. I am still window shopping. I went to my LHS yesterday and saw a few Harley. The IMAI 1/12 Harley seems nice to me but quite expensive (Cdn $49). Also saw a Academy 1/10 Harley for only Cdn $36. I guess the Academy one is a clone of the Tamiya. I am afraid the 1/6 Harley from Tamiya is out of the question ($$$). Once again thanks guys.

I saw a few Harley kits the other day when I was snooping in my LHS. Don't Revell America do a range of HD kits?
I've put a couple of Tamiya's GP bikes together & they are an absolute gem of a kit to build - if you ever get bitten & want to push on from the Hogs. A mate of mine built Tamiya's Hayabusa & it was jaw dropping. The paint finish was so good, you could almost see your reflection. I think there's a build over on Hyperscale of it.