Incidently, with your Sony A300, welcome to the Mind (Heart and Soul) of Minolta!
For lenses, i'd echo what Drewski said and use what you have for now - unless IQ (Image Quality) is utterly disappointing.
So, with that, use your camera as much as you possibly can for 1 month or 2 if needed. After that time look at all your photo's and see what types of photos you most usually take. Determine your needs then.
If you're really set on buying a "new" lens, the 1st lens i'd suggest is an old Minolta AF 70-210mm f4 lens known as the "Beercan" coined by J. Greely in a 1999 article here:
Minolta Autofocus Camera Lenses - photo.net
It is a 70-210mm f4 telephoto lens and for it's price ~ $200, it is superb. It is useful for telephoto as well as portraits. I have had a few of them and sold a few, but always have one with me when using my digital gear. I've also used it with some Kodak UC400 color film and the images it produces are astounding - especially for a $200 lens. Be careful if shopping on e*Bay for one b/c lotsa folks want to lump their sub-quality in the with the Beercan, but always have a variable aperture and not the constant f4 the Beercan has. There are absolutely no other variations of it- none. Read more about it here:
Minolta*AF 70-210 F4 (beercan) Reviews at Dyxum.com
and here:
Is there another version of the 70-210mm "Beercan"? - Photo.net Sony/Minolta SLR System Forum
Regardless of what anyone tells you there is only 1 "Beercan". For a little more reach, it does have a stable-mate, referred to as the "Big Beercan" and it is a 75-300mm f4.5-5.6. and it is pretty well regarded, but not as much as the Original Beercan.
Another gem is the Minolta 28-135mm f4-4.5 (i am awaiting delivery of mine as well so can't personally verify it's legendary abilities). It is reputed to be as sharp as the "Beercan", but has a better range. It's not without compromise, so check out the reviews here:
Minolta*AF 28-135 F4-4.5 Reviews at Dyxum.com
I am planning on it being my walk-around lens and have been researching for about 2 years. I've reviewed hundreds of images from it and by all accounts, it also lives up to it's reputation.
If you find yourself really wanting a higher quality wide-angle, there are a number of lenses available, but wide-angle is pretty spendy.... For example, there is a Carl Zeiss 16-35mm f2.8 full-frame lens due to be released shortly and is exclusively available for Sony that will likely be priced around $1,800. If that is a bit over the top, there are also the APS-C format 11-18mm lenses by Minolta/Sony/Tamron as well as a well-regarded Carl Zeiss APS-C 16-80mm. Additionally, there are the well-regarded Sigma lenses such as the 15-30mm f3.5-4.5, or 12-24mm f4.5-5.6, or super-wide 10-20mm f4.5-5.6. Tamron has recently released a 10-24mm lens, though its too new for much opinion to be formed of it's quality.
This turned into a little more than i initially planned, but hope it's helpful nonetheless.
frank